AutomationEditor: fix grid scrolling and non-4/4 time signatures

Closes #283 and closes #303.
BUT, please don't put this in 1.0.0, at least not without thorough testing! There might be some place where ```DefaultTicksPerTact``` should not be changed to ```MidiTime::ticksPerTact()```.

If the time signature has an odd denominator, there will be two grids visible, one from the time signature and one from the quantization, not lining up nicely.