fixes #124

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.

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).

This reverts commit 4b807229.

fixes #394

- ESC closes the popup.
- Up/Down arrows for navigation.
- BackTab for backwards navigation.

fixes #301
fixes #302

Fixes #287

* Fix pop-up not showing when less than max 

* Select suggestion by pressing Enter

fixes #40

* Refactor widget items to use same interface

* Support audio, video, generic file for pasting

* Add utils function for human readable file sizes

* Set correct MIME type for media messages

This change also determines the size of the upload once from the
ContentLengthHeader, rather than seeking the QIODevice and asking for
its size. This prevents any future trouble in case the QIODevice is
sequential (cannot be seeked). The MIME type is also determined at
upload once, rather than using the QIODevice and the underlying data
inside.

* Allow for file urls to be used as fall-back

This fixes an issue on macOS which uses `text/uri-list` for copying
files to the clipboard.

fixes #228 

fixes #132 

fixes #24

The color scheme of nheko obeys the default color theme of Qt
(i.e. the system theme). It uses a Qt stylesheet to accomplish this,
which means replacing the color theme with a custom theme would only
be a matter of writing a new style sheet and loading it into the app.