Fixes for Mac OS X

Makes the following adjustments for proper operation on Mac OS X:

- Configuration dialog's tab widget adjust its size and then asks the
  window to also adjust its size when the active tab changes, which
  fixes the middle tab appearing smashed in vertically
- Browser Generator displays a summary window after closing the dialog,
  and sets its ownership to the Browser window rather than the dialog
  itself (avoids a lock-up problem where Mac OS X thinks that the dialog
  is still open, preventing the user from interacting with Anki).
Fix for shortcut conflict on Browser window

While in the Browser window's Editor mode, two shortcuts were active
simultaneously (i.e. the Browser Generator menu shortcut in the Edit
menu and the more local Editor Generator button along the toolbar),
which means that only the Browser Generator was usable. Given the
context, the user probably actually wants the Editor Generator.

This adjusts the action menu shortcut to become disabled when focus is
gained on the Editor field, and re-enabled when focus is lost, allowing
the more "local" shortcut to prevail.