Add a shortcuts section to Preferences

Currently the only way to customize hotkeys is to edit the programs "accels.scm" file.
Suggesting to add a "Shortcuts" tab to Preferences.
This would allow users to assign custom shortcuts using a GUI.
Similar to how programs such as Thunar and Terminal offer.

ARROW KEYS
Arrow keys should be allowed to be customized, currently they are hard coded (cannot assign arrow keys in accels.scm file).
Would have add this update to do so #137

MORE THAN ONE HOTKEY FOR COMMONLY USED ACTIONS
I personally like to be able to set more than one hotkey for commonly used actions.
For example similar to how in Thunar you can set more than one hotkey for cut, copy and paste actions. If this gets added to Ristretto, if possible could someone add the ability to set more than one hotkey for the more commonly used actions.
List of examples of what actions to offer more than one shortcut for can be found here if anyone is interested #140.

Thank You