vapigen: Support transfer_ownership for delegates
vapigen: Support transfer_ownership for delegates

Add attribute to support owned delegate return values.

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