When using Thunderbird landscape, add option for fullscreen

[fytdyh]

When using Thunderbird in landscape, add option to enable Fullscreen and hide status bar. In order to be more efficient and productive. 

[Jon]

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[kbauer]

This, but please without the landscape limitation.

For some uses, e.g. webcomics, every screen pixel matters. While it is more pronounced in landscape mode, where modern 21:9 displays leave but a thing stripe for actual content even before websites pile their own horizontal elements on top, it would also help a lot with portrait mode.