Refactor color scheme related code

Currently, we have two functions that update the color scheme for a
client - updateColorScheme(QString) and updateColorScheme(). Even though
they both share the same name, they do different things. The first one
sets the specified color scheme, while the other determines the color
scheme preferred by the client and assigns it.

This change refactors the color scheme initialization code so we no
longer need those two methods. The setColorScheme() method sets the
specified color scheme, and the preferredColorScheme() method returns
the color scheme preferred by the client. Sub-classes of AbstractClient
can override the preferredColorScheme() method in order to add support
for platform-specific color scheme protocols.

The end result: color scheme related code is a bit more comprehensible.