Allow to take screenshots

Regular Screenshots would be a first step towards !181 (closed) . I find myself using wlroots/_build/examples/screenshot rather often.

We could do that outside phosh in a separate process.

Relevant protocol https://github.com/swaywm/wlr-protocols/blob/master/unstable/wlr-screencopy-unstable-v1.xml

TODOs after !782 (merged):

• Capture all outputs
• capture single window
• allow to select screen area