previously the code that tried to activate the whiteboard failed because it checks `can_modify_game_state` which checks for `!synced_context::is_unsynced`
F5 causing the editor to crash - the editor saves temporary files out to disk which don't have an extension, so give them the proper extension
The add-on selection dialog showing non-add-on folders in the add-ons folder - only show files with a _main.cfg
The key word of course being "start". This PR changes the editor by default to work at the add-on level instead of in its own separate scenarios and maps directories. The goal is to make the editor more useful generally, but also specifically to make it much easier for players to distribute content they create using the editor:
* When they click the Editor button on the main menu, they will first be prompted to choose an add-on (or mainline), or to create a new add-on.
* When saved, if the scenario cfg is in the format previously generated by the editor, it will be converted to the new format and to use the [multiplayer] tag, the map_file attribute, and have the map data saved to a separate .map file.
* Relatedly, the editor now knows how to handle scenarios with the map_file attribute at all (which yes, does mean that currently wesnoth's editor doesn't know how to load its own mainline scenarios from their cfg).
* When opening the file chooser dialog, it now defaults to their selected add-on's directory.
If they choose to create a new add-on, then the editor creates for them:
* a basic but functional _main.cfg.
* an empty achievements.cfg (at this point mostly just so they might see it at some point and realize achievements exist, but ideally an achievements editor dialog could be created eventually).
* an empty _server.pbl file.
* a proper add-on directory structure containing the standard set of folders (maps/, scenarios/, units/, utils/, images/, etc).
Additionally, as an initial proof of concept for actually using this new add-on level functionality, a new Add-ons dropdown has been added to the editor's top bar, with a pbl editor option. This allows populating the blank _server.pbl file as well as editing an existing _server.pbl. There is also an option to change the add-on's ID, which will update the add-on's folder name and _main.cfg.
Misc other changes:
* The ability to add a recruit list to a side has been added back as a text box on the edit side dialog. While admittedly this doesn't allow players to select units from within the editor itself, it does set the actual side's recruit list (rather than a specific unit's extra_recruits), will show the user what the current recruit list for the side is (which the previously removed implementation didn't show anywhere), and correctly sets the faction as Custom so that the recruit list is used.
* When saving an old-style editor scenario, everything that can be triggered via [event] is either moved to a start event with a specific id attribute. Exceptions to this are [time], [side], and [side][village]. This is done so that the editor can know (for the most part) what things are actually its own to safely replace. As such, aside from the three previously mentioned tags plus the start event, any other WML added to a scenario by a UMC author is preserved rather than being overwritten - the editor no longer replaces the entire contents of the scenario file.
* The editor no longer writes out cfg files missing the top level scenario tag. If it doesn't find one or this is the first save of a new scenario it defaults to [multiplayer], but otherwise it will properly handle finding [test] or [scenario] as well.
* Requires that map files have the .map extension and scenario files have the .cfg extension. Also it assumes that .map files do actually only have map data in them and the .cfg files do actually have valid WML in them. I understand that this is not a limitation it had previously, but I don't think this is an unreasonable thing to require.
* Addresses part of #7667 by just not using regex for figuring out the map_data attribute value.
Additionally, it is not possible to change the currently selected add-on without going back to the main menu, clicking the editor button, and choosing a different add-on. This is intentional - I don't want to deal with having multiple add-ons open at the same time. If someone feels really strongly otherwise, then they can implement that themselves later.
