network: Ensure WirelessSecurity type is initialized

When calling for the wireless security widgets, the code
simply assumes that the corresponding GType is initialized.
This may not always be true, which leads to a nasty crash
every time e.g. we open the network connection editor dialog.

This commit fixes that by introducing a new standard macro
wrapping wireless_security_get_type(), and ensuring the type
is initializing when calling wireless_security_init(), thus
protecting every code path from this crash.

This commit also makes CePageSecurity use the new macro for
better legibility.

https://bugzilla.gnome.org/show_bug.cgi?id=785581