Player Change adds Player without making a choice

[henkmosseveld]

I made a new file
changed the player to a three-string instrument
added a new system
entered the caret in the new system
tried to add a new Player with Player Change
There is no choice (except none) to be made in the pop-up menu
So I enter OK without making a selection, (not even none, just the blank field):

PTE still enters a Player 1

[Canaanabolaanan]

If you do this process, it no longer adds Player 1 to the measure. But it does consider a player change to have occurred. You can click the "Player" menu and the Player Change will be checked despite nothing displaying in the measure. Maybe the "Ok" option should be disabled until a proper player for the measure is selected. Otherwise, just "cancel" should be accessible.

[cameronwhite]

IMO it should be valid to have an empty player change, since to fix #146 we'll need to clear the active players which have the wrong number of strings. I think we maybe just need to have a visual indication that there are no active players, instead of having the player change be invisible.