ListBoxHeader() can return false.  Also made ListBox() only use public API.
ListBoxHeader() can return false.  Also made ListBox() only use public API.

seanmiddleditch/imgui,nProtect/imgui,shaggytwodope/imgui,cmaughan/imgui,zho7611/imgui,nmlgc/imgui,emoon/imgui,cmaughan/imgui,xythobuz/imgui,mikesart/imgui,wflohry/imgui-addons,cmaughan/imgui,GHF/imgui,cmaughan/imgui,gyakoo/imgui,kylawl/imgui,thennequin/imgui,elect86/imgui,Pagghiu/imgui,bagobor/imgui,shaggytwodope/imgui,ggtucker/imgui,wasikuss/imgui,tom-seddon/imgui,usagi/imguixx,xunmengfeng/imgui,burningreggae/imgui,benoitjacquier/imgui,ocornut/imgui,twigletguy/imgui,aimotive/imgui-glfw,seanmiddleditch/imgui,tom-seddon/imgui,benoitjacquier/imgui,mikesart/imgui,thennequin/imgui,burningreggae/imgui,dougbinks/imgui,ghassanpl/imgui,xythobuz/imgui,andresv/imgui,ghassanpl/imgui,neshume/imgui,andresv/imgui,sakishum/imgui,luiseduardohdbackup/imgui,redblobgames/imgui,wflohry/imgui-addons,rxantos/imgui,thevaber/imgui,GHF/imgui,wangshijin/imgui,luiseduardohdbackup/imgui,bkaradzic/imgui,redblobgames/imgui,wangshijin/imgui,aimotive/imgui-glfw,gyakoo/imgui,bkaradzic/imgui,ocornut/imgui,usagi/imguixx,Psybrus/imgui,nProtect/imgui,elect86/imgui,bagobor/imgui,Psybrus/imgui,twigletguy/imgui,neshume/imgui,Flix01/imgui,dougbinks/imgui,ocornut/imgui,bkaradzic/imgui,Pagghiu/imgui,nmlgc/imgui,Flix01/imgui,xunmengfeng/imgui,ggtucker/imgui,tpoechtrager/imgui,thevaber/imgui,rxantos/imgui,Flix01/imgui,sakishum/imgui,emoon/imgui,rmoorman/imgui,zho7611/imgui,nProtect/imgui,ggtucker/imgui,rmoorman/imgui,wasikuss/imgui,kylawl/imgui,tpoechtrager/imgui