Instead of hard coding the desktop files, let's try to use what the
user actually uses.
* Browser mapped from text/html mime type
* File manager mapped from inode/directory mime type
* Email client mapped from message/rfc822 mime type
Only systemsettings is "hard coded" as there is no mime type to map.
The thumbnails are not changed, though. But they are so small that
it's hardly recognizable anyway. For future it could be considered to
map against appdata to get a better screenshot.
BUG: 310622
REVIEW: 122170
4a260c6f42