Remove unnecessary "owner" property of Field objects.

The owner property was only ever used for Species, and it is far simpler
to check if the field is a species field than it is to check if it's owner
is Species
Move InstanceInfo into the data package.

All of the other classes which subclass Model are there, so it seemed
appropriate
Show properly formatted dates for display on TreeInfoDisplay
Add date picker for editing date fields

Shows a button in place of a text box, and opens a dialog with a date
picker when it is clicked.

Fixes #112
Move ids from strings.xml to ids.xml
Merge pull request #131 from maurizi/fix/112/date-planted

Add ability to edit date fields