calendareventsplugin: Add support for alternate dates and sub-labels

Sub-labels are required to show alternate calendar in the calendar
applet.

Two new signals are added:

1. `subLabelReady`: Emitted when the plugin has loaded the sub-labels.
2. `alternateDateReady`: Emitted when the plugin has loaded the
   alternate dates.

`SubLabel` struct contains yearLabel, monthLabel and dayLabel, and the
plugin can specify the priority among Low, Default, High and Urgent for
a SubLabel.

CCBUG: 429882