gutter: use connect-after for the view's draw signal

This is more correct since we want to draw on top of what gtk
draws on the gutter. It is necessary with latest gtk since
it started clearing the background with the css specified colors
Release 3.13.1
Post-release version bump
automake: Add AM_TESTS_ENVIRONMENT as a keyword

https://bugzilla.gnome.org/show_bug.cgi?id=732146
CompletionContext: dispose() gracefully on buffer change

The buffer can change during dispose(), as explained in the code.
The commit here doesn't fix all the issue, but dispose() should now run
correctly if the buffer changed.
File loading and saving

New public classes: GtkSourceEncoding, GtkSourceFileLoader and
GtkSourceFileSaver.
There are also new properties in GtkSourceBuffer.

https://bugzilla.gnome.org/show_bug.cgi?id=721016
Move some enum types to gtksourcetypes.h
build: rename helper library private -> core
encoding: comment unused functions
GtkSourceEncoding documentation
Some code improvements
GtkSourceFile, the come back
Move mount_operation_factory to GtkSourceFile
Add missing GI annotation
FileSaver: add setting: ignore invalid chars
File saving: check if the file is externally modified

Store the modification time in GtkSourceFile, and update the value on
each file loading and saving.
FileSaver: flags instead of boolean properties
FileSaver: use IGNORE_MODIFICATION_TIME flag
Make GtkSourceMountOperationFactory introspection-friendly

There was a GI warning.
Make load_async() and save_async() introspection-friendly