Improved install routine

[the-phinet]

This commit set contains a number of fixes and changes. I can split them into multiple pull requests if you'd like

Fixes:

• The deletion temp folders 'deps' and 'aseprite' could result in loss of user data. This have been fixed by using a unique temp folder
• The script cleans up after itself, even if it exits early
• Any commands that touch the file system are checked for success, and will exit early if they fail. This addresses Error handling #4

Changes:

• Moved the output folder to the location specified by XDG
• Added aseprite to the user's path for command line execution
• Script now automatically installs the official desktop file