GbDocumentManager: add gb_document_manager_get_unsaved_documents()
GbDocument: add misc helper functions to document

Adds get_mtime(), get_read_only(), and is_untitled() interface functions.
GbDocument: add helper to get seconds since known mtime.
GbEditorDocument: implement GbDocument interface functions

Adds helper functions for mtime, is_untitled, and get_read_only()
GbCloseConfirmationDialog: import modified close dialog from Gedit

This renames symbols and uses our document abstraction instead of
GeditDocument.
workbench: add plumbing for save confirmation dialog

This doesn't actually perform the saves (which we need to do while
blocking the main loop). That will come next once we plumb async
saving and save-as.