Introduce zen mode

Most of the time developers write code in a particular file and don't wont anything else to bother them. There's a "FullScreen" feature which's made for these reasons.

VScode has gone further and created a ZenMode, which allows to maximize dev's focus on the code. It doesn't seem to be a "hardly-implementing" feature. But one the same hand I find it a great one.

I've photoshoped a concept look of this in Qt Creator.