Defined more theme variations and styleboxes for those variations to work around an issue where external editor themes would apply incorrect margins to certain buttons. This should eliminate clipping/alignment issues while a custom theme is in use.
Also simplified the step where margins are copied over thanks to @Rindbee 's suggestion
Default shortcuts use the first or second letter of each word.
This also adds a new shortcut to toggle the last opened bottom panel.
On editor startup, this defaults to the first panel in the list
(which is the Output panel).
This can be used to quickly see how recent a development build is,
without having to look up the commit date manually.
When juggling around with various builds (e.g. for benchmarking),
this can also be used to ensure that you're actually running the
binary you intended to run.
The date stored is the date of the Git commit that is built, not
the current date at the time of building the binary. This ensures
binaries can remain reproducible.
The version timestamp can be accessed using the `timestamp` key
of the `Engine.get_version_info()` return value.
