Add a prefix variable to all the places that use <C-p> as a prefix

[LunarWatcher]

<C-p> has turned out to be a fairly problematic prefix, with several plugins (and a Vim built-in) using just <C-p>.

However, I cannot keep switching the keybinds whenever there's a conflict. Instead, the prefix should be replaced with an easily configurable variable, to let people make their own decisions on what prefix makes sense (and let them make them with whatever limits their plugins place on free keys).

(In case someone else wants to pick this up before I get to it; contributions to this particular issue are on the develop-4.0.0 branch, not on the master branch)