texthistory: add GtkTextHistory helper

texthistory: add GtkTextHistory helper

The GtkTextHistory helper provides the fundamental undo/redo stack that
can be integrated with other text widgets. It allows coalescing related
actions to reduce both the number of undo actions to the user as well as
the memory overhead.

A new istring helper is used by GtkTextHistory to allow for "inline
strings" that gracefully grow to using allocations with g_realloc(). This
ensure that most undo operations require no additional allocations other
than the struct for the action itself.

A queue of undoable and redoable actions are maintained and the link for
the queue is embedded in the undo action union. This allows again, for
reducing the number of allocations involved for undo operations.