Drag&Drop import

[georgehrke]

When dragging a calendar file from the desktop to the calendar app

• [ ] everything but the calendar list should get a grey overlay
• [ ] a temporary Import into new calendar calendar item should appear
• [ ] user should be able to drop ics files onto any writable calendar or the Import into new calendar calendar item

[georgehrke]

• [ ] What to do when a user drags multiple ics files into the calendar? Options when dropping onto the Import into new calendar item:
• import all ics files into one new calendar
• import each ics files into a new dedicated calendar
• show two temporary calendar items instead:
  • Import into one new calendar
  • Import into {n} new calendars

[georgehrke]

cc @jancborchardt

[georgehrke]

also cc @raghunayyar