If needed, add LibCurl as HTTP manager

What happens:
- SFOS 3.0 will bring Qt5.9 support, until then we don't have support for HTTP/2 in QNAM. SSE isn't supported at all.
What was expected:
- In case we want HTTP/2, SSE, ... on Qt5.6 we should add LibCurl as HTTP Manager.
- Or write your own SSE wrapper https://github.com/btubbs/sseclient
- LibCURL SFOS: rinigus/mapbox-gl-qml@e8ef2a1
How to reproduce:
1. N/A
Environment:
- Sailfish OS version: 2.2.0.29
- Sailfish OS hardware: Xperia X, Jolla 1

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