Don't generate wxEVT_CHAR_HOOK events while the mouse is captured.

This prevents the parent TLW from interfering with the keyboard handling of
the window that captured the mouse which very often needs Escape for itself to
cancel the capture.

In particular, this fixes the problems with Escape closing the entire dialog
containing the controls instead of closing just the combobox drop down or a
popup menu in wxMSW.

Also modify wxGTK for consistency and update the documentation.

Closes #12952.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67191 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775