Merge lp://qastaging/~charlesk/indicator-datetime/lp-1426519-add-tag-for-menuitem-activation-url into lp://qastaging/indicator-datetime/15.10

If the "X-CANONICAL-ACTIVATION-URL" x-prop is defined in the VTODO or VEVENT, use it for url delegation.

Sibling 15.04 MP: https://code.launchpad.net/~charlesk/indicator-datetime/lp-1426519-add-tag-for-menuitem-activation-url/+merge/253997

If the "X-CANONICAL-ACTIVATION-URL" x-prop is defined in the VTODO or VEVENT, use it for url delegation.

Since no clients are using this key yet, a little white-box testing is necessary:

Steps
1. in clock-app, create a repeating alarm
2. $ stop indicator-datetime
3. $ pkill -f evolution-calendar-factory
4. Edit ~/.local/share/evolution/tasks/${some-unique-epoch-id-generated-by-evolution}/tasks.ics:
   After the line "BEGIN:VTODO^M", insert a new line:
   X-CANONICAL-ACTIVATION-URL:http://www.canonical.com/^M
5. $ start indicator-datetime
6. pull down the indicator menu
7. click on the alarm that you edited in step 4

Expected result:
Browser should launch to http://www.canonical.com/