Add a hotkey to swap D-pad and left analog stick keys

Some games use D-pad for in-game movement & menu selections while others use left analog stick for that (leaving D-pad for items usage, map etc routines). Instead of re-defining keys for every game it's better to have a "swap" hotkey to switch between D-pad and left analog stick keys.

This patch is used in ROSA & OpenMandriva linux but likely it's worth of merging upstream.
I don't like that this is persistent.  Shouldn't we just map differently when intercepting the press and translating?

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