fixes #214

fixes #97
fixes #51

it was causing the full screen image overlay to appear on the "primary
display" vs. the actual display nheko is running on.

removing the move() call makes the overlay follow the window.

No idea, why apply visitor doesn't work with temporaries?

Pagination could get stuck, if the messages request failed.
Section height seemes to have been calculated to late, which would make
some section overlap the next message in some cases. Fix that by doing
the height calculation manually.

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.

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

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.

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

When opening images in the full-screen overlay, there is now
a download button next to the close button.  This button utilizes
the same functionality as the right-click->save image button.