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.
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.
Basically reverts the last font related commits since pointSize isn't
as reliable as pixelSize.
Also some layout values (margins, spacings) have been moved out to Config.h.
