buttonrebinds: add support for keyboard rebinding

uses existing Key handling

the relevant config format is for kcminputrc:

[ButtonRebinds][Keyboard][Razer Razer Orbweaver]
87=Key,X

where the device name is from libinput list-devices, 87 is the Qt key code for Key_W and X can be a QKeySequence.

This feature will enable us to remap keyboard-style devices such as gaming mice and gaming keypads.