[nemo-qml-plugin-calendar] Refactor plugin to use DB thread

No changes to the public API.
[nemo-qml-plugin-calendar] Remove obsolete code
[nemo-qml-plugin-calendar] Prefix source files with SRCDIR

Enables including src.pro from other pro files
[nemo-qml-plugin-calendar] Add test framework
[nemo-qml-plugin-calendar] Tests for NemoCalendarManager::isRangeLoaded(...)
[nemo-qml-plugin-calendar] Tests for NemoCalendarManager::addRanges(...)
[nemo-qml-plugin-calendar] Packaging for tests
[nemo-qml-plugin-calendar] Remove obsolete yaml file
[nemo-qml-plugin-calendar] Cache events in NemoCalendarManager
[nemo-qml-plugin-calendar] Enable moving events between notebooks
[nemo-qml-plugin-calendar] Tests for NemoCalendarEvent