Add option to enable gui with command line

[Joonalai]

Expected behaviour
There should be a command line option --qgis_enable_gui to enable gui. This is important to for example to disable gui in local env, but to enable it in other environments.

Current behaviour
There is only command line option to disable gui --qgis_disable_gui.

Describe alternatives you've considered
Using env variable QT_QPA_PLATFORM=offscreen and QT_QPA_PLATFORM= to disable and enable gui.