The counter is incremented after each new generation and reset
whenever any cell on the board is toggled. Resizing the board
does not reset the tick count.
This is not only easier to comprehend code-wise and avoids some function
overloads, but also makes resizing the board while preserving game state
*a lot* easier. We now no longer have to allocate a new board on every
resize, we just grow/shrink the individual row vectors.
Also fixes a crash when clicking the board widget outside of the drawn
board area.
