mobile infrastructure changes

[Dirk Hohndel]

Hi Everyone,

I finally was able to merge something I have worked on for quite a while... we can now build the mobile app against Qt 5.15 without the UI completely falling apart.
As side effects of that we had to switch to a newer Kirigami version, v5.76; this is the last one to support Qt 5.12 - which we technically /could/ ignore, but there wasn't anything in 5.77 that seemed worth abandoning the ability of many of the developers who are on Ubuntu (which still has 5.12 in 20.04) to build mobile-on-desktop.

I pushed new Android and iOS builds to the beta channels on both stores - I'd love testing and feedback on those before I turn them into official releases.

I have tried to test this reasonably well - but as we have seen time and again, my testing never gets anywhere close to being comprehensive.

Thanks

/D