Properly install GTK+ theme depending on version

Currently, even if the theme was updated, CMakeLists.txt would still
pick data from an old "gtk-3.0" directory, instead of the newer
"gtk-3.18" (for GTK+ <= 3.18) or "gtk-3.20" (for GTK+ 3.20 or newer).

This patch fixes it by setting a runtime option ("-DWITH_GTK3_VERSION")
which is set to 3.18 as default (same behavior as before). If one sets
-DWITH_GTK3_VERSION=3.20 or anything higher, CMake will install the
GTK+ 3.20 version.

Thus, (fellow) packagers, bear this in mind: the next version of Plasma
(5.7) will require -DGTK3_VERSION=3.20 added to your CMake setup command
if you use GTK+ 3.20, otherwise, nothing will be needed.

Both Breeze and Breeze-Dark have been adjusted for this.

This was done as opposed to having a version check to not require the
GTK+3 development package just to check the version.

CCMAIL: plasma-devel@kde.org
CCMAIL: kde-distro-packagers@kde.org