New icons, screenshots and illustrations from design

Named things with an eos prefix to keep our assets obvious in the
big list. Also added a secondary prefix of the type of image--
control center screenshot, desktop screenshot, keyboard illustration.
So that if we say update the control center significantly it will be
obvious which assets need updates.
[endlessm/eos-shell#4235]
New content from Nuritzi

With a little help in the mallard xml from yours truly.

Not merging this with the earlier content update commit, as a lot
of translations were done on the previous commit, so it will keep
things clearer to stay separate

[endlessm/eos-shell#4235]
Update the pot file
[endlessm/eos-shell#4235]