libnm-gtk: add support for TTLS/EAP-MD5 and TTLS/EAP-GTC (bgo #729671)

Add the simple EAP methods MD5 and GTC as inner/phase2 methods for TTLS.

These methods must be inner *EAP* methods too, unlike PAP/CHAP/MSCHAP,
so we need to update the code to ensure that when they are selected,
the editor/applet puts them into NM_SETTING_802_1X_PHASE2_AUTHEAP instead
of NM_SETTING_802_1X_PHASE2_AUTH.  Take the opportunity to clean up
all the booleans passed into eap_method_simple_new() while we're here
so that we don't have to add yet another boolean argument.

Inspired by a patch from Pali Rohár <pali.rohar@gmail.com>