Start using g_object_notify_by_pspec()
From the last commit message:
`g_object_notify()` actually takes a global lock to look up the property
by its name, which means there is a performance hit (albeit tiny) every
time this function is called. For this reason, always try to use
`g_object_notify_by_pspec()` instead.
Using g_object_install_properties()
is actually a best-practice imho.
I did a small check and added a log after every call to g_object_notify() in gimpobject.c, and in the loading screen only, it is already called a whopping 6185 times. So that's 6185 times we're uselessly locking/unlocking to look up the property name :-)