Another small release that fixes some issues and releases another iteration of the editor (still early WIP).

• Editor
  • Fixed a crash when returning from the Editor the menu, and then selecting editor again.
  • Escape now exits the editor, unless a modal dialog is active in which case it closes that dialog instead.
  • Levels and 3DOs are listed in the Asset Browser, but functionality is somewhat limited.
  • Added multi-select.
  • Added the ability to force selected assets to use an alternate palette for testing.
  • Added export selected - allowing for assets to be exported and PNGs to be generated where appropriate for viewing.
  • Added central error handling and modal message box functionality for errors/warnings.
• General
  • Linux: Make RtMidi optional - this allows for OPL 3 and SF 2 synth but disables system midi (@mlauss2).
    • This should make it easier to compile in some cases.
  • Replace DeviL with SDL Image to make future Linux packages easier (@mlauss2 / @luciusDXL).
  • Small PNG issues fixed (@mlauss2).