Themes from the user config where not restored correctly.
Improve error handling when no themes are found
in case of broken installations.
Cleanly differentiate between theme "id" (currently complete basename of
theme file) and theme "displayName" (as specified as a property in the
theme file).
Remove convoluted broken code that tried to allow using an absolute file
path for a theme on the command line and require themes to be installed
either in Qt Creator globally or in the user settings path.
In general stream line the code.

Task-number: QTCREATORBUG-15113
Task-number: QTCREATORBUG-15233
Change-Id: I014a4314e8bea27422ed4c42462cf16f4220698b
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>