Draft: kcms/mouse: Make it possible to set keyboard bindings with any keys

We want the user to be able to bind a mouse button to a key that doesn't exist on your keyboard.

(bunch of other fixes can be split into another MR)

The idea is roughly like this: to supplement the key sequence recorder control with a less interactive but all-encompassing list of Qt.Key_* enum values and a grid of keyboard modifiers.

We can't use this huge UI everywhere, because the implicit assumption about KeySequenceItem is that it is small — which isn't really the case with 4-key-combination monster sequences, but at least vertically it's small. An alternative would be to implement some form of modal dialog, but we can't assume that a parent window would have enough space for a dialog either (think of Plasma panel configuration popup — focus shortcut). That leaves us basically with no options but to develop something new and custom without the burden of any prior assumptions.

Also, it is unusual for this type of component to be used to specify keys which are not present on user's keyboard.