cmake: Clean up icon-related build options
Remove -DWITH_BREEZE, -DWITH_BREEZE_DARK and -DWITH_OXYGEN options
from CMake. Instead, provide two new options that should make things
a bit clearer:
-DWITH_BUNDLED_ICONS now controls if bundled icon themes should be
installed/embedded. Defaults to ON to ensure that required
icons are available unless explicitly disabled (e.g. by a
package maintainer, who should then make sure that a dependency
to Breeze/Oxygen exists).
Disabling this option saves less than 2 MB of disk space.
-DWITH_OXYGEN_ICONS controls whether the Oxygen icon theme should
still be supported. This affects both the Oxygen-themed
Quassel-specific icons and the bundled icon theme. Defaults to
OFF when building against Qt5, ON for Qt4.
Adapt code accordingly, and update INSTALL.