app-tile: Completely remove star ratings

Completely rip out star ratings from GsAppTile in order to make more
room for text, until we have a way to automatically generate the needed
data. They were already disabled in commit 044ef3d but still hooked up;
this removes them from the ui file as well.
Improve tile layout in the category view

... as per the designer guidance.

https://bugzilla.gnome.org/show_bug.cgi?id=712158
trivial: Remove an unused plugin
Add support for the AppStream <languages> tag
trivial: Parse AppStream with both <licence> and <project_license> tags
Add the concept of 'kudos' to an application

Applications can do certain things if they achieve various kudos.
Add a kudo if an application is available in the users language
Add a kudo if an application is featured or recommended