bookmarks: Use available dialog space for bookmark list
requested to merge michaelweghorn/accerciser:michaelweghorn/bookmarks_use_available_space into master
Use the existing space for the tree view of the existing bookmarks in the "Edit bookmarks" dialog, rather than giving it only the same height as the three buttons next to it.
Improves the user experience for the dialog for managing bookmarks ("Bookmarks" -> "Edit bookmarks...") when several bookmarks are present, since less scrolling is needed and it's easier to find a specific bookmark.
Sample screenshots:
Before:
After:
