GbEditorDocument: track read-only and file mtimes

This can be used to discover changes from the underlying file while
editing a document.
editor: dont allow editing when buffer is read only
document: add missing declaration
GbEditorFrame: check for external modifications when focused
GbEditorDocument: reset modified state when loading file
GbEditorDocument: add gb_editor_document_reload() helper
GbEditorView: show infobar warning when document changes externally

This should look familiar, it's intended to follow the design that
gedit took.