Retrieves context around the insertion point.
Input methods typically want context in order to constrain input text based on existing text; this is important for languages such as Thai where only some sequences of characters are allowed.
This function is implemented by emitting the GtkIMContext:
GtkIMContext:retrieve_surrounding signal on the input method; in
response to this signal, a widget should provide as much context as is available, up to an entire paragraph, by calling
set_surrounding. Note that there is no obligation for a widget
to respond to the retrieve_surrounding signal, so input methods
must be prepared to function without context.
location to store a UTF-8 encoded string of text holding context around the insertion point. If the function returns true, then you must free the result stored in this location with g_free.
location to store byte index of the insertion cursor within
true if surrounding text was provided; in this case you must free the result stored in *text.