add key bindings capabilities to the kcfg-viewer

[iFrostizz]

Currently, the kcfg viewer is entirely mouse-driven It would be a nice change to be able to run through this viewer and be able to move using only the keyboard.

Some potential changes ideas:

h,j,k,l move in any tab (select next node, scroll down in a window, ...)
Ctrl + d go down a page
Ctrl + u go up a page
Ctrl + w trigger a window change (h,j,k,l) to actually move
By typing a number, we could store it in a buffer and replicate x times the move action.

UI suggestion by @anvacaru keep the highlight focus on the node that is currently selected, even when the mouse isn't hovering it anymore (same using the keyboard)

[iFrostizz]

maybe also replace h by f for fold as it will interfere with the "go left" key binding

[iFrostizz]

Mostly done by #578
Can be reopened if missing features are requested