Further updates for the appdata file.
Further updates for the appdata file.

 - Correct the value of the id
 - Reorganise a bit and add some whitespace to make it more readable
 - Add help URL, point bugtracker URL to the wiki instead of b.g.o
 - Add categories, launchable and provides

tshikaboom/geary,tshikaboom/geary,tshikaboom/geary