gtask: Improve task names used internally within GLib

And improve them externally, where not otherwise set, by setting them
from the function name passed to `g_task_set_source_tag()`, if called by
third party code.

This should make profiling and debug output from GLib more useful.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

gio/gasyncinitable.c

@@ -87,6 +87,7 @@
  *   GTask *task;
  *
  *   task = g_task_new (initable, cancellable, callback, user_data);
+ *   g_task_set_name (task, G_STRFUNC);
  *
  *   switch (self->priv->state)
  *     {