[Qt] Implement QQuickMapboxGL

[tmpsantos]

A QML Item exposing an interface compatible with QtLocation Map.

At the moment the plan is to use QQuickFramebufferObject which will set our minimum Qt version supported at 5.2.

The Qt5 build bot was already upgraded #4827.

/cc @brunoabinader

[brunoabinader]

We've progressed substantially the past week, and now QQuickMapboxGL already has a working example:

We are keeping up API parity with Qt's Map QML Type in our QQuickMapboxGL QML Type, based on QQuickFramebufferObject + QMapboxGL. A QML example application is provided and can be built/executed via $ make run-qt-qml-app

QQuickMapboxGL implements the following:

• Qt Map Type-like properties
  • Fully supported: minimumZoomLevel, maximumZoomLevel, zoomLevel, center, error, errorString, visibleRegion
  • Replaced with style: activeMapType, supportedMapTypes
  • Not needed/missing: gesture, plugin, mapItems, copyrightsVisible, color (depends on Runtime styling API #837)
• QQuickMapboxGL-specific properties: style, bearing, pitch