Add the About dialog to the project manager

[Calinou]

The About button is located in the bottom-right corner of the project manager.

This allows removing the copyright notice from the window title (which looked a bit ugly in comparison to other applications).

It turns out EditorNode doesn't need to be referenced in EditorAbout for the theming overrides. With my changes, the dialog looks the same in the editor as it did before this PR.

Preview

[aaronfranke]

Something weird happens to the top of the About page if I scroll a little bit:

[Calinou]

Something weird happens to the top of the About page if I scroll a little bit:

This bug is unrelated to this PR, as it also happens in the editor's About dialog, not just in the project manager. It's present in master since the Complex Text Layouts update at least (if not before).

[bruvzg]

To work with the macOS system menu (which always include "About" menu item), ProjectManager should handle NOTIFICATION_WM_ABOUT notification:

void ProjectManager::_notification(int p_what) {
	switch (p_what) {
...
		case NOTIFICATION_WM_ABOUT: {
			_show_about();
		} break;
...
	}
}

[akien-mga]

Thanks!

[akien-mga]

Would need a dedicated PR to cherry-pick, it's too much bother cherry-picking changes to the project manager from 4.0 to 3.x.