Removed last remaining traces of the "update:" binding option, since it's not needed - all Data changes from the openHAB end come to us as Commands.
Moved the bind-value handling, for "special" bind-values (?,??,?++,?--), into MiosConnector, so the values can be correctly URL-Escaped as needed prior to transmission to the MiOS Unit.

Sliders now correctly handle INCREASE (+1, via "?+"), DECREASE (-1, via "?-") and the Command's value (via "??") correctly.
It also substitute in the Item's current-value as the bind (using "?"), since we're now registered to get the ItemRegistry.