Automatically dismiss unread notifications when the window regains
focus.

fixes #111
fixes #68

fixes #134

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.

[ci skip]

[ci skip]

Fixes #64

This also fixes the transmission of mis-typed commands as messages,
fixes inability to send messages that start with a command, and does
some initial work towards automatically resizing the input field to fit
the input message.