Database clear works, unlink works

Changed from database file delete to table dropping and remaking
Deleting from table now checks full path, not just loc
Added confirm dialog to linking
Added return if no selection in unlink
Unlink now changes ObjectListViews and sqlite database accordingly
Symlink function now uses the current link_path if a folder is already
linked, and the original_path if it isn't. This way for previously
linked folders, link_path can store the previous link.
Removed Folder.set_link_loc(self, link_loc) since it was unneeded.