AppImage support for passing of Categories for .desktop file

[mackenza]

• Version: 6.4.0

• Target: Linux AppImage

I would be very useful to be able to pass a list of Categories as options for build.linux key.
The Categories is a string in the form (LibreOffice Calc example):
Categories=Office;Spreadsheet;X-Red-Hat-Base;X-MandrivaLinux-Office-Spreadsheets;
https://specifications.freedesktop.org/menu-spec/latest/apa.html#main-category-registry

brackets-userland/brackets-electron#65

I will try and work on a PR for this but wanted to get it logged here just in case someone else can knock it off faster than me ;)

[develar]

Relates: #641

[develar]

PR is welcome. Update: I will do it, it will be faster :)