Fix overloading of "source" and "target" terminology in GBinding

GBindingTransformFunc called its arguments "source_value" and
"target_value", but in the transform_from function of a bidirectional
binding, "source_value" comes from the target object, and
"target_value" comes from the source object, which quickly gets
confusing if you need to use g_binding_get_source(), etc, in the
function.

Of course developers can call their transform function arguments
whatever they want, but many will copy from the headers/docs to start
out, so use less confusing names here ("from_value" and "to_value").

Also, fix the documentation to describe the bidirectional case
correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=709440
gmain: make g_source_add_child_source() thread safe

g_source_add_child_source() releases the context lock before attaching
child_source to context. And this causes trouble if parent source is
blocked and g_main_dispatch() manages to lock the context mutex and call
unblock_source() before child_source gets attached to context.
To fix this we call g_source_attach_unlocked() before releasing the
context mutex.

https://bugzilla.gnome.org/show_bug.cgi?id=711064
gcancellable: don't use g_cancellable_connect() in GCancellableSource

g_cancellable_connect() is documented as calling its callback only
once, but GCancellableSource should trigger every time the cancellable
is cancelled.

https://bugzilla.gnome.org/show_bug.cgi?id=710691
https://bugzilla.gnome.org/show_bug.cgi?id=711286