Detect KDE5 and fallback to KDE4UI there

Change-Id: I8da53d7b81d28b0051be96c0c4ee0a29d8ed8360
Reviewed-on: https://gerrit.libreoffice.org/12209
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>