Make qgis_show_map more usable

[Joonalai]

Expected behaviour qgis_show_map marker should be more usable when developing tests. Here are few improvement suggestions:

• Move statement https://github.com/GispoCoding/pytest-qgis/blob/d81a06d0316d95719cee80edd400a950e3e460d6/src/pytest_qgis/pytest_qgis.py#L349 to _show_qgis_dlg method to make the layers appear in correct order and with correct visibility during the test and not just in teardown.
• If timeout is se to zero, do not perform anything in _configure_qgis_map method, just close the qgis_parent
• Add option to show the qgis_parent in full screen mode. On Windows it is not trivial to open the qgis_parent in correct size