Windows CI dependency upgrade (#2847)

* Windows CI: Use OSG 3.4-experimental for 0.46

* Update compiled Windows CI dependencies

Only built and pushed so far, still need to try making full OpenMW
builds with them as well.

* Update missed Bullet version number

* MyGUI uses RelWithDebInfo for Release builds now

* Update Windows CI dependencies, switch Qt install

* Fix aqt retrieval and setup

* Make aqt install output slightly nicer

* Bump to Qt 5.15 for VS2019 support

* Fix FFmpeg and Qt install parts

* Fix OSG plugin DLL copying

* Add CMake flag for double-precision bullet

* Roll back 2019 to Boost 1.71 for CI

* Move aqt into unpack step, to allow manual install