Add doubleClick handlers on empty slots

[alxmiron]

This PR is a part of #563 issue

New:

• onDoubleClickSlot handler
  Callback fired when empty calendar cell was clicked twice.
  (date: Date, e: SyntheticEvent) => any
• onDoubleClickAllDaySlot handler
  Callback fired when empty calendar all-day cell was clicked twice.
  (date: Date, e: SyntheticEvent) => any

Changes are similar to onSelectSlot handler

[jquense]

On selectSlot already handles the click case, maybe we can extend that for dblClick without the pointer-events? I want to avoid that css if possible in the core library for browser support. I'm less concerned about it in addons since they are additions.

[alxmiron]

Ok, I'll look into Selection object, what we can do with it.
Note, that pointer-events have good browser support

[alxmiron]

@jquense See another solution in #575