Make picker controls description in the manual less useless.

It's not really clear what potential benefit could library users extract from
the knowledge that the picker controls behaviour is "highly
platform-dependent".

Try to be flighty more helpful.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76712 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Add a page containing the screenshots of all controls to the manual.

It can be helpful to see all the controls at a glance, especially if you don't
know which control is needed.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76713 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775