Allow more than one controller to attach OnFocus/OnFocusLost functions

Trying to do this would previously have the second one silently overwrite the
first one's.

We don't currently have this in lazygit, but I ran into the situation once
during development, and it can lead to bugs that are hard to diagnose.

Instead of holding a list of functions, we could also have added a panic in case
the function was set already; this would have been good enough for the current
state, and enough to catch mistakes early in the future. However, I decided to
allow multiple controllers to attach these functions, because I can't see a
reason not to.

pkg/gui/controllers.go

@@ -21,7 +21,7 @@ func (gui *Gui) Helpers() *helpers.Helpers {
 // the lower in the list the keybindings will appear.
 func (gui *Gui) resetHelpersAndControllers() {
 	for _, context := range gui.Contexts().Flatten() {
-		context.ClearAllBindingsFn()
+		context.ClearAllAttachedControllerFunctions()
 	}
 
 	helperCommon := gui.c