Properly disconnect signal handlers added with e_signal_connect_notify*()

This is a follow-up for the previous commit, where e_signal_connect_notify*()
functions had been added. Due to a different callback and user data being
attached to the 'notify' signal, the g_signal_handlers_*() functions do not
work properly, thus these e_signal_connect_notify*() functions need
a different way for a signal handler disconnect.

A side-change was done in e-settings-web-view-gtkhtml.c, checking for a real
key change from GSettings.