refactor Page::getFormWidgets() to use unique_ptr

Refactor Page::getFormWidgets() to return
std::unique_ptr<FormPageWidgets> so to avoid
possible leaks in API users who forget
to delete object.
7 jobs for glib_memleak_unique in 3 minutes and 35 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #7979040
build

00:03:07

passed #7979045
build_android

00:00:38

passed #7979041
build_clang_libcpp

00:03:20

passed #7979044
build_clazy

00:03:34

passed #7979043
build_mingw64_fedora

00:01:53

passed #7979042
build_ubuntu_18_04

00:02:16

passed #7979039
clang_format

00:00:50