Moving through every focusable widget by tab/shift-tab is very slow. This commit introduces the concept of groups--widgets that can be skipped through via ctrl-tab/ctrl-shift-tab.
Right now this only includes containers where all children are focusable, but I'll expand the definition of groups over time to make navingating to large areas of the UI easier..
When we unfocus, wait a second. If we don't have focus after the timer elapses, warp the mouse to the global coordinates.
This seems to restore some semblance of focus whenever it is unset, while not breaking the case where a focus is set and the mouse suddenly jumps, thus changing the focus again.
Here I add a `ScreenReader` `Node` which, when added to the `SceneTree`, should read the contents of any supported `Control`. I also do a bit of cleanup on filenames, as well as breaking the editor plugin support into its own class.
