KEYMAPPER: Separate the virtual mouse from the global keymap

[ccawley2011]

The actions for the virtual mouse are currently added to the global keymap. Since this keymap has the lowest priority however, actions in other keymaps override it if it has an action that uses the same default bindings. This is a problem with some platforms like OpenDingux which don't always have an analog stick, meaning that it's necessary to map the virtual mouse to the D-pad, and then individually unmap all actions in other keymaps that use the D-pad, which is somewhat impractical in the long term.

One way of approaching this would be to move the virtual mouse actions to a separate keymap that has the highest priority, and then maybe allow engines that don't use mouse input to enable or disable the virtual mouse automatically to allow per-engine keymaps to use the left analog stick.