Enable the gtk3_kde5 and kde5 VCL plugins as discussed
in ESC all of 2019-05-02.
Also drop the "--disable-gtk3" switch so gtk3 is built
as well (which is enabled by default).

Change-Id: I65230255f8db6a7de992c6f5e9f9892a72436349
Reviewed-on: https://gerrit.libreoffice.org/71865
Tested-by: Jenkins
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>