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.

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.

Fixes "sent an audio file" replies, that were actually replies to text
messages.

I'm not sure, if that is the right way, but Qt doesn't really have a way
to format custom localised dates, so I tried to find the closest
approximations to what we currently have.

Relates to #69

Respect system styling
Increase size of emojis (to remove empty space)
Add hover effect (partially adresses #41)
Less hardcoding of sizes
Use emoji font (color)

Mentions are now an '@' icon in the upper right.

UI Popup is now a smaller dialog.

Still lots of work to be done here.

If the QMimeData contains an image, it actually has a mime type of
application/x-qt-image. At least in some cases accessing the image/*
data returns a 0 length array. Accessing the data via ->imageData works
however. So we use that as our accessor and pass it to the preview
dialog.

fixes "Attribute Qt::AA_EnableHighDpiScaling must be set before
QCoreApplication is created."

Add a RoomListItem-like button that opens a dialog
containing all of the messages that would result in a
highlight from the server (for example, the user is mentioned,
or @room is mentioned).

This is VERY rudimentary and will be completely reworked in the future
to take advantage of the existing TimelineView class, instead of
using a dialog like it does now. The button to show the mentions
also needs work.

Update to mtxclient 0.3.0