[SailfishOS] Provide MapBox map for SailfishOS

[micuintus]

See explanation: https://lists.sailfishos.org/pipermail/devel/2017-November/008121.html

API: https://github.com/rinigus/mapbox-gl-qml/blob/master/api.md#include-statements
https://www.mapbox.com/mapbox-gl-js/style-spec/#sources

Example:
https://github.com/rinigus/poor-maps/blob/gl-release/qml/Map.qml

Might also be a cure for our current iOS mapbox dilemma

Problem: This API is clunky (can we use font based icons or does it only support PNGs?)

When will Jolla the SailfishSDK to allow QtLocation?!

[jmastr]

I managed to compile and link it for iOS (not macOS, linking did not work), but one needs to INCLUDE directories from the mapbox-gl-native git repository and also link against the library?!

-INCLUDEPATH += src/plugin
-    
-LIBS += -lqmapboxgl -lz -L/opt/gcc6/lib -static-libstdc++
+INCLUDEPATH += \
+    src/plugin \
+    /Users/julian/dev/qt5/qtlocation/src/3rdparty/mapbox-gl-native/include \
+    /Users/julian/dev/qt5/qtlocation/src/3rdparty/mapbox-gl-native/platform/qt/include
+
+LIBS += -L/Users/julian/V-PlaySDK/V-Play/ios/lib -lqmapboxgl -lz -L/usr/lib -lcurl

(-lqmapboxgl is already required).