vapigen: Support owned_get metadata attribute for properties
vapigen: Support owned_get metadata attribute for properties

Fixes bug 592462.

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