[Enhancement] Allow us to press the SPACE BAR to quickly close a picture preview (for YT video preview, see description)

[Contribucious]

As in "Echofon for Mac" and probably other popular clients, I used to press the SPACE BAR with my left thumb to quickly close a picture preview. So, it can be good to add that. Clearly more convenient than using the default "Close window" keyboard shortcut (which is often a combination of keys like Alt+F4, Ctrl+Q, Ctrl+W, …).

For the YouTube videos, I personally think that it's better to let the SPACE BAR acting as play/pause as the default behavior (which is not currently the case by the way, because the player don't have the focus by default, which is needed for this behavior).
And rather closing the preview with the mouse. But it can be discussed.

P.S.: It's often little things like that, that completely make you love a client or no. Because people have habits and it's very hard to switch else, or in an uncomfortable way then! So, it can be good to add all these little details (more useful than one might think), especially when we can found classic/generic patterns used by many popular clients, but rarely used on the alternative clients (especially on the Linux platform!). So, you've a clearly good way to make you famous with all of these little details that many Linux clients often neglect! Especially considering that you've (specially you) many switching users coming from the Mac world, by the Elementary canal/way. :)