#28 speedup enter of "Period" events

[roger lindberg]

The user can't decide the default settings used when the 'Create new Event' dialog is opened.
This makes it more cumbersome to enter period events, because the period checkbox has to be checked each time. That means that it's not possible to use shortcuts to enter a new period event without having to check the period checkbox.

[roger lindberg]

Suggested solution: Create a new menu alternative 'Create Period Event...' that when used, opens the 'Create Event' dialog with the period checkbox checked.

[roger lindberg]

Suggested solution: Remember the checkbox setting from last time the Event editor dialog was opened to create a new event.