Desktop GUI: add a "Preferences" window to the "Spirit" menu

[GPMueller]

This would be handy for controlling default behaviour of the GUI, such as:

• whether to change the geometry of the system to that which is read in from a file (see also Core: improve config loading (in particular IO_System_From_Config) #572)

[GPMueller]

This has been implemented on the feature-imgui branch (PR #583) as "File->Settings". Currently, it contains the dark/light mode switch, so the proposed geometry switch should still be added.

Note: the IO API would have to be improved to allow the "update geometry" setting (either add a bool update_geometry(...) or add functions with which to query the size of systems in files - the former should suffice).

[GPMueller]

Merged into develop with commit 728b478.