wpa_gui: Sort frequency and signal numerically in the scan results dialog

Signal strength was sorted lexically rather than numerically, which
put "-100 dBm" before "-50 dBm" if sorted in descending order.
This change fixes that. It also treats frequency in the same
manner, preparing it for the IEEE 802.11ah.

Signed-off-by: Adrian Nowicki <adinowicki@gmail.com>

wpa_supplicant/wpa_gui-qt4/wpa_gui.pro

@@ -34,6 +34,7 @@ HEADERS	+= wpamsg.h \
 	wpagui.h \
 	eventhistory.h \
 	scanresults.h \
+	scanresultsitem.h \
 	signalbar.h \
 	userdatarequest.h \
 	networkconfig.h \
@@ -45,6 +46,7 @@ SOURCES	+= main.cpp \
 	wpagui.cpp \
 	eventhistory.cpp \
 	scanresults.cpp \
+	scanresultsitem.cpp \
 	signalbar.cpp \
 	userdatarequest.cpp \
 	networkconfig.cpp \