Chording should "press" surrounding unopened cells

[Namdrib]

Idea: if detecting that chording is happening, loop through unopened neighbours, change their image
If the mouse leaves the current cell, unpress the neighbours
If the mouse enters a cell, and is chording, press the neighbours

Potential problem: like the cascading "opens" and mine reveals (as in #13) this may cause the cells affected by both the leaving and entering step to become unpressed and pressed again.