Adapt to "QGIS4" directory in QGIS 4

[kannes]

Following qgis/QGIS#64752, the plugin will need some non-trivial updating to support both QGIS3 and QGIS4 profiles 😢

Since the QGIS 3- > QGIS 4 -> QGIS 3 round trip corrupts the QGIS settings ini, we need to silo away the QGIS 4 settings in order to avoid user profile corruption.

We do this now by a one-time copy of the loaded user profile from the QGIS3 path to a new QGIS4 path on startup. This is a lossless settings migration, with all user settings maintained. Note that it is ONE TIME ONLY, and after that initial migration any changes made to the profile in QGIS 4 won't be reflected in QGIS 3 and vice versa.

[kannes]

Unfortunately I won't have time/budget to work on this particular plugin in the foreseeable future. I will ask if we (WhereGroup) can allocate some budget though, unless someone else is happy to jump in.

[kannes]

I find it hard to decide if it would be worth working on cross-version compatibility (lots of work) or if it would be fine to have a final QGIS 3 version and simply adjust the paths in the code to QGIS 4 for any subsequent versions (little work).

Users might find the Profile Manager especially useful when juggling with QGIS 3 and QGIS 4 profiles in the transition period between versions but who knows really...