Port the Qt5 viewer demo to use the lazy outline item API (even if for loading…

Port the Qt5 viewer demo to use the lazy outline item API (even if for loading all items up front for now).
6 jobs from qt5-lazy-outline-api in 13 minutes and 20 seconds (queued for 4 minutes and 29 seconds)
Status Job ID Name Coverage
  Build
passed #58286
build

00:05:43

passed #58291
build_android

00:01:04

passed #58287
build_clang_libcpp

00:05:40

passed #58290
build_clazy

00:05:02

passed #58288
build_gcc_49

00:07:35

passed #58289
build_mingw64_fedora

00:04:28