The interface is taken from Qt/KDE, but the implementation is different,
because the Qt implementation depends on some Qt internals.

requires Qt5.12+

Size is fixed for now, otherwise the Video output ends up somewhere...

I suck at sizing so the images in the message are currently hardcoded to
300 pixels in width...

Text selection over multiple items doesn't work yet

Mentions are now an '@' icon in the upper right.

UI Popup is now a smaller dialog.

Still lots of work to be done here.

This reverts commit 4b807229.

fixes #235

fixes #204
fixes #255

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.

* Validate both inferred and explicitly entered server addresses by attempting to call the /versions endpoint
* If the domain from the mxid fails validation, try prefixing it with 'matrix'
* Only show server address field if address validation ultimately fails