Multi window

[tcanabrava]

This is a WIP for the multi-window feature.

Things that we need to keep in consideration:

• ThemeManager receives only one Qml Engine, it will need to keep track of the std::vector<std::unique_ptr
• I can't test because I don't have the devices
• singleton classes could potentially bug the software since each window will need to keep track of it's own data.

Still, I can open two instances of the window from pingviewer within the same PID, and I can block two different applications running.