libxml bindings: Improve coverage
libxml bindings: Improve coverage

This commit adds the following functions:
- xmlNextElementSibling
- xmlFirstElementChild
- xmlChildElementCount

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