Save the focusType instead of getting it on every keydown

Save the focusType instead of getting it on every keydown

This is simpler and more efficient. Also, we were basically already storing the
current focusType, since the button uses it to grey out when a text is focused.

Config API changes:

- `match` objects no longer have a `focus` property. It has been replaced with
  the `likelyConflict` property. (Theoretically a breaking change.)
- `vim.focusType` is now available and contains the current focus type.
- The `focusTypeChange` event now passes only the current `vim` object to
  listeners, instead of an object of data. Use `vim.focusType` to get the new
  focusType. (Theoretically a breaking change.)