Bug 1037405 - implement the doorhanger prompting the user to share a screen/window, r=Gijs.

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extra : rebase_source : 394a06b14a7561ea1f43d3c7a8ce52ad4a4fe447
Bug 1037405 - adapt the label of the main action in the screensharing doorhanger prompt, r=Gijs.

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extra : rebase_source : 4cf00c04c99814c2bb040ff65d3d37793ac59976
Bug 1037405 - add new icons for screensharing doorhangers, r=Gijs.

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extra : rebase_source : 0795c223b752322c6843b2c64441749722050679
Bug 1037405 - implement the screen sharing urlbar indicator, r=Gijs.

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extra : rebase_source : ae477beaae5553db887589bd5ce314fb2a63c2ef