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.

fixes #446

fixes #447

fixes #437

When the object is destroyed the connections will be removed
automatically by Qt.

fixes #433

fixes #418

fixes #416

fixes #393

fixes #305

The latter will not always return a valid pointer.

fixes #363