Update the stash manager to show more information about stashes.

- use the newly defined method in the previous commit to get information
  about stashes
- show the stack position, branch & parent commit description for each
  stash item
- show the date-time of each stash as a tooltip
- make the stash dialog resizable (now that it shows more information,
  which not always fits)
- fix tooltip for the drop button
- separate the close button from the others (instead of the drop button)
- make the stash list extend to the bottom of the dialog