new dialog from menu: edit task

[WillForan]

1. add menu item for "edit task".
2. give a list of tasks to edit.
3. upon selection reuse the AddTask.py dialog to edit a task. Populate default values with those already in the the row for this task.
4. if "edit", use update instead of insert.

Be careful with settings (which is currently null for all tasks) -- only change what the gui knows about -- don't clear everything. That is if settings has a foo key before edit, it should have foo after, at least until the addtask gui knows about foo.