* SDL2 builds of the game now set their app window icon.
* Create/Edit Level window is updated to show a tabbed UI to create a
new Level or a new Doodad. The dedicated main menu button to create a
new doodad (which immediately prompted for its size) is replaced by
this new tab's UI.
* Edit Drawing/Play Level window is more responsive to smaller screen
sizes by drawing fewer columns of filenames.
* Bugfix: the Alert and Confirm modals always re-center themselves on
screen, especially to adapt between Portrait or Landscape mode on a
mobile device.
