Bug 685786 - EWebView: Signal handlers never disconnected

Both EWebView and EMailDisplay listen for "changed" signals from a
GSettings instance, passing itself as the 'user_data' to the signal
handler e_web_view_update_fonts().  But in both cases the signal
handler was left connected after EWebView and EMailDisplay were
finalized, resulting in the signal handler receiving a dangling pointer.

Not using g_signal_connect_object() here because of the unresolved
reference leak issue in GObject.  The GSettings instance is likely
cached internally and lives well beyond EWebView and EMailDisplay.