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

Quoted replies now include matrix.to links for the event and the user.
UI Rendering has been (slightly) improved... still very WIP.
Restructured the reply structure in the code for future usability
improvements.

Add placeholder UI for showing replies in the text entry widget.
Existing quoting capability has been removed (Temporarily), as
it was replaced with the new reply capability.  Replies sent from
nheko do not currently appear correctly in the timeline (this
will be fixed in a future commit).

Parsing html as xml has inherent problems, most notable there are many
matrix clients that don't escape ampersands in urls of mx-replies, etc.
(See issue #18)

This also removes the replacement of <mx-reply> as it isn't strictly
needed.

Also the QRegExp is replaced with the Qt5 QRegularExpression for
perfomance and because it supports lookahead and lookbehind.

I'm pretty sure that the original code also replaced href="" with
href=\"\", which was probably wrong, but I'm not to sure about that.

Fixes #18

(requires mtxclient updates as well).