Require the description when listing installed applications

Fixes the same issue as the previous commit, but for the installed apps
view.
Drop unused icon cache
appstream plugin: Always use the full app ID

Now that GsApp no longer has the concept of short application ID after
commit 5e4992f95c2af81ca909fae60b6749543c1b02f8, make sure we use the
full app ID including the .desktop suffix when constructing GsApps.

This fixes Nautilus not getting listed as an unremovable system app.
details: Move install button to a more prominent place

Jimm Hall's usability testing revealed that people often miss the
install button in the headerbar. This commit moves it to a more
prominent place in the dialog content area, as per Allan's mockup.

https://github.com/gnome-design-team/gnome-mockups/blob/master/software/version2/software-app-uninstalled.png
Overview: Rename the section headings as per designer guidance

Rename "Picks" -> "Editor's Picks" and "Featured" -> "Featured
Application".

https://bugzilla.gnome.org/show_bug.cgi?id=727998
overview: Show popular games as a separate section in the landing page

https://bugzilla.gnome.org/show_bug.cgi?id=727998
overview: Hide games if we don't find enough to fill in all tiles

Users might have almost all available games installed and we might not
find enough games to fill in all 6 tiles. Just skip the games section in
that case.
appstream plugin: Implement getting popular apps from a specific category
overview: Change the recommended apps category each day

Instead of just showing recommended games, rotate through the audio,
games, graphics and office categories.
overview: Pre-create empty tiles for rotating recommended apps
details: Move the spinner and the pending label to the content area

... following the Install / Remove button move.
trivial: Rename a variable
overview: Fine-tune tile spacing

... to avoid vertical scrollbars when maximized on a 1920 x 1080
monitor.
modulesets: Split out non-gnome modules to default.xml
modulesets: Add a few more non-gnome games
details: Refresh the page after remove confirumation dialog

This fixes an issue where cancelling the remove confirmation dialog
could leave the add-on checkboxes in wrong state.

https://bugzilla.redhat.com/show_bug.cgi?id=1128354
trivial: Silence a compiler warning
Updated Greek translation