feat: MultiCount Interface

[wegreenall]

Adds a MultiCount interface. This extends the MultiSelect to allow for (integer) numerical counts. Rather than doing anything complicated, the count is incremented/decremented with the right and left arrows, and incremented/decremented by ten using shift and right, left arrow respectively. I stuck close to the MultiSelect paradigm to minimise the difference from old code in large commits for new features, to make it easier to compare.

If accepted I will add an appropriate README paragraph (it felt rather brash otherwise).