* Add new class QtInstanceComboBox as a weld::ComboBox
  implementation using a native QComboBox. Implement
  some of the most important and straightforward methods
  and trigger an assert for all others for now.

* In QtBuilder::makeObject, handle the "GtkComboBoxText"
  case and create a QComboBox for that.

* Implement QtInstanceBuilder::weld_combo_box
  to return a QtInstanceComboBox instance.

Change-Id: I2ac2d0f55a948ea2f090c81096484e22899ddcbc
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/175443
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins