[Appointment widget] Choose calender folder to view appointments from

Current appointment widget only checks the calendar folder in the default store.

Expected

• In widget settings create a field name: "Calendar:" and a button to select a calendar folder, called "Select calendar". (Similar to select folder in rules dialog)
• Upon click a new window pops-up with all calendar folders
• Widget title should be changed to: Appointments of <calendar name>+<- location> e.g. "Appointments of calendar - Henk de vries".
  if your own calendar is configured don't change the title, keep "Upcoming appointments"
  if you select another calendar folder inside your own store. Do title: "Upcoming appointment of <calendar>
• Scope all opened calendar folders with read permissions (Can be own, public or shared)

Possible error handling:

• When chosen folder is not available, due to deletion or lack of view permissions, fall back to no folder. Empty view text should be added: "Please select a calendar via widget settings"