[build][qt] Add support for using qmapboxgl as a proper CMake dependency

[PureTryOut]

This way other CMake projects requiring it can just do the usual:

find_package(QMapboxGL REQUIRED)
target_link_libraries(<target name> PUBLIC QMapboxGL)

~This depends on #16394 to actually install the target we're linking against so it includes it's commit. This needs rebasing when #16394 is merged.~ This is now considered the continuation of #16394.

[rinigus]

Please consider this PR as further development of https://github.com/mapbox/mapbox-gl-native/pull/16394

If accepted, it will fix #16328 for Qt platform.

[PureTryOut]

@tmpsantos can I get a review on this?