[Feature] Add more keybindings

[pidgeon777]

More very useful keybindings to be used in the treeview panel/popup:

• set the symbol under cursor as new root for callers
• set the symbol under cursor as new root for callees
• preview the destination without jumping on it and closing the popup/panel*

*for this it would also be very good to add an automatic preview option: when navigating the popup/panel treeview items, when placing the cursor on them, the corresponding line should be shown in the buffer or, eventually, on a completely new small preview box as done in other plugins.