Rules editor: Function reference can't be hidden or removed

[joschi]

The function reference on the right side of the rules editor (System / Pipelines / Manage rules / Edit) takes up much of the available screen estate which could be used for a larger text field (see #229).

While the function reference is useful at some times, it's usually just a dead block of wasted screen estate on the rules editor.

Instead of displaying the function reference all the time, we could place it in a concealable sidebar, only display it on demand if a button was clicked (or a hotkey was pressed, e. g. ?), or even open it in a new window on demand.

Small window, large sidebar

Medium-sized window, large sidebar

Large window, large sidebar