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).

Colors are generated asynchronously now and the TimelineItem is
updated when the color generation finishes.  This allows the UI
to stay responsive while new colors are being generated.

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.

Restored the emoji picker, but it now falls back to the system
instead of forcing Emoji One.  The allows users to user the
picker for convenience, but doesn't enforce the emoji style on them.

This reverts commit 4b807229.

Add initial loading of qt jdenticon plugin:
https://github.com/redsky17/qt-jdenticon

Currently, the library's functionality has not been integrated
into the rest of nheko.  Next step is to add a configuration
item in the User Settings and use the plugin to generate
avatars for users without their own picture.  These avatars
should be cached in the Cache object.

User colors are now stored in cache.  This is consistent
with other similar variables.  I think there's a bug
right now where it doesn't properly refresh colors
for the TimeLineItem when the theme is changed.

When user is mentioned (via matrix 'highlight_count'), inactive
rooms will use a different color for the notification circle than
when only general unread messages exist.