Better support for having the same key bound to different actions

[dorkster]

While working on #1783, there were some cases where I wanted the same physical key/button bound to multiple actions. One example of this would be gamepad L1/R1 being used for both WidgetTabControl navigation as well as being used for Main1 and Bar0 respectively. But I found that determining the correct context and blocking unwanted actions was too complex with the way we're currently handling input.

So for now, I'm sticking the current scheme of one action per physical key/button and putting this issue on the back burner.