move the help browser files into their own folder

This is the start of a refactor to make all the help code more
easy to follow.
split help_button out of help.hpp header
add help_impl.?pp files (guts of help.cpp)
split off help_browser from help_impl
split off help_text_area from help_impl
fixup headers of new files using iwyu
split off help_menu from help_impl
Revert "add help_impl.?pp files (guts of help.cpp)"

This reverts commit c1366ad9d9829ff38d71109246c9d96932f0075f.
Revert "split help_button out of help.hpp header"

This reverts commit af34dc39312e026a8c7031cc1750dfe86f9dba8b.
Revert "move the help browser files into their own folder"

This reverts commit a93a3fda2b547496eaf56a0532377a701a382c7a.
Merge branch 'move_help_browser'