build: Qt 5.15.x

Opening this for visibility. Apart from it probably being time to use a newer version of Qt for releases, an update is required so that we are using a version which has a C++17 mode.

There's a lot of commits here, because I've made some changes (like rebasing our patches), that aren't strictly necessary. There are also some changes, like bumping the minimum required macOS version, that will likely happen separate, as part of the C++17 migration. So this change will end up being a bit smaller.

The biggest change here is that Qt no longer has internal xcb libs, so we need to build the requirements ourselves. This also means some additional build-time tools, such as yacc (from bison) are also required; to build xkbcommon.

I've tested this on Ubuntu; although it'd be great if more people could test with their particular brand of Linux desktop.

Cross-compiled Windows and macOS builds compile and run, but the GUI currently looks a bit broken on both. i.e macOS seems to look a bit like an older version of Windows:

I haven't had enough time to fully investigate (also why there's some WIP commits), so if anyone wants to dig in and see what Qt has changed since 5.9.x feel free.