Try to reduce memory usage by reusing avatar pixmaps

Mentions are now loaded from the cache instead of
directly from the web request.  Mentions are also
properly saved to the cache now (instead of as empty
strings).  Still lots of tweaks left on this feature.

On first launch, before the user has configured any settings,
check the value of the QT_QPA_PLATFORMTHEME environment var.
If it is set, use the system theme as the default instead of the
light theme.  This fixes #72.

Cache user mentions when they are retrieved from the server.
This logic currently isn't being utilized by the UI.  Additionally,
the app should use a 'since' value to only get mentions newer
than those stored in the DB, to avoid excessive web requests.

This will be implemented in a future commit.

Currently translated at 97.0% (191 of 197 strings)

Translation: Nheko/nheko
Translate-URL: http://weblate.nheko.im/projects/nheko/nheko-master/de/

Currently translated at 100.0% (197 of 197 strings)

Translation: Nheko/nheko
Translate-URL: http://weblate.nheko.im/projects/nheko/nheko-master/en/

Currently translated at 100.0% (195 of 195 strings)

Translation: Nheko/nheko
Translate-URL: http://weblate.nheko.im/projects/nheko/nheko-master/en/

Mentions are now separated into 'this room' and 'all rooms'
tab., which allows the user to filter on the current room
if they desire.  Should add additional logic in the future
to show which room the mention was in the for the 'all rooms'
view.