Left+right clicking should expand like middle click

[Namdrib]

Some notes on behaviour:

• this should only trigger the opening of neighbours if the number of adjacent flags is equal to the cell's number, AND

• this should trigger if both left and right mouse buttons are down and one of them is let go

• this is still valid if both left and right leave the intended CellIcon and return over it again

Rough plan:

• record left/right is down upon mouse press
• record left/right is up upon mouse leave
• record left/right is down upon mouse enter if it is still down
• trigger open surrounding upon left/right release if the other is still down

[Namdrib]

First normal left click buggy - doesn't do anything

[Namdrib]

Note: After chording, treat all mouse buttons as not being pressed. e.g. left click down -> full cycle right -> treat as left and right back up

[Namdrib]

Bug: the game currently doesn't remember the state of mouse clicks/presses between CellIcons.
e.g. In the following image, right-mouse-down at (1) then (while right mouse still down) left-mouse-down at (2) doesn't correctly trigger a chording action at position (2).