Separate select and install parts in themes install window.
Allow using the themes installation dialog as selection dialog separately from
installing themes. For this the installation is now triggered separately and
can be told to not to do the installation on Ok button. In this case the dialog
is selection only, and the Ok button is changed to Select. The installation
itself is still done in the class but started by calling the install() method
separately.
Change-Id: I856db8204788302b2b539e6d8283f73cb354f033