Allow for remapping Volume buttons in ES

[dfilskov]

Issue description

Please make it possible to map volume to any buttons and any input to the volume buttons.

[theflyingape]

While the external "scripted" methods documented on wiki do work, it makes no sense why native integration does not exist that allows es_input.cfg to do them itself out of the type="keyboard" entry, i.e.,:

		<input name="volumeup" type "key" id="1073741952" value="1" />
		<input name="volumedown" type "key" id="1073741953" value="1" />
		<input name="quit" type="key" id="1073742093" value="1" />

... the media volume keys are self-evident, but as for another useful example, a functional ES "quit" function mapped to a keyboard with a "Home Page" button (OS typically launches default web browser) would be nice too. Certainly, there are others that can be explored, but these 3 are essential.

Thanks for your consideration.