Moves all platform specific code from CommonSalLayout into the
platform specific plugins. This way the vcl library won't depend
on the Qt5 libraries and the Qt5Font header can be moved into the
qt5 VCL plugin.

While at it, switch the CommonSalLayouts font reference from the
FontSelectPattern to the LogicalFontInstance and also add the
harfbuzz font handling to the instance.

Change-Id: Ida910b8d88837ea949a2f84394ccc0cfae153060
Reviewed-on: https://gerrit.libreoffice.org/47408Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Khaled Hosny <khaledhosny@eglug.org>