Opening projects via command line args

Found some issues while drawing that needed to be fixed, and I added the ability to open a file via command line args. You can now set Retro Sketch as the default application to open the save files.

Features

• Ability to open project files from the command line

Fixes

• Fixed an issue with animation preview crashing when creating new tracks and swapping between them and opening animation preview
• Fixed an issue with clearing of selection sometimes being 1 pixel down when undoing