Fix missing const with virtual properties
Fix missing const with virtual properties

tjol/vala-tjol,flobrosch/vala-fixes,themiwi/vala,spidermonkdust/vala,rvu95/vala,Distrotech/vala,tjol/vala-tjol,GNOME/vala,themiwi/vala,spidermonkdust/vala,Jc2k/vala,rvu95/vala,jessevdk/vala,severinh/vala,themiwi/vala,rvu95/vala,frida/vala,jessevdk/vala,lovebug356/vala,frida/vala,Distrotech/vala,flobrosch/vala-fixes,xdave/pvala,GNOME/vala,flobrosch/vala-fixes,phako/vala,spidermonkdust/vala,playya/vala,Distrotech/vala,GNOME/vala,severinh/vala,frida/vala,GNOME/vala,kamanashisroy/aroop,tjol/vala-tjol,frida/vala,jessevdk/vala,frida/vala,severinh/vala,phako/vala,Jc2k/vala,kamanashisroy/aroop,lovebug356/vala,playya/vala,kamanashisroy/aroop,playya/vala