cmake: Clean up icon-related build options

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.