[invader-edit-qt] Add scenario editing

[SnowyMouse]

~~A full 3D scenario should be implemented into invader-edit-qt, as I feel that having a separate tag editor for it is redundant.~~

A full 3D scenario editor should be implemented in some form, probably as a standalone tool (see https://github.com/SnowyMouse/invader/issues/134#issuecomment-769388029).

#133 should be completed, first, of course.

[SnowyMouse]

Also, this tool should not necessarily be tied to #33 / #80. The stock HEK has script compilation tied into the scenario editor, Sapien, and this is really inconvenient if you are not interested in editing using the scenario editor.

It can be an option to compile scripts, but a separate command line script compiler should still be an option.

[SnowyMouse]

Having a standalone scenario editor instead of integrating it with invader-edit-qt may actually be a good idea as it wouldn't depend on Qt which, as of late, has been precarious for a number of open source projects.