Warn when using `weak' modifier where it is not applicable
Warn when using `weak' modifier where it is not applicable

Based on patch by Jiří Zárevúcky.

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