[10_SOMFY.pm] Add new reading 'move_state'

[uwekaditz]

This could be used to display the reaction to a command (webCmd or widgetOverride) in the FHEM web interface, since otherwise you would not get a Notify-Event indicating that the movement was started or stopped.
The initial state after starting a command (move=on or move=off) is supressed to see a change in the devStateIcon if the reading move_state is used to alter the state.

[viegener]

There is no problem in adding a reading that is indicating the movement is finished, but removing intermediate reading changes will be affecting users that rely on intermediate updates of the position reading.
Would just adding the move_state reading also be sufficient?