Don't use too big width for wxHtmlHelpWindow navigation panel.

Don't use too big width for wxHtmlHelpWindow navigation panel.

The width of the navigation panel in wxHtmlHelpWindow could become arbitrarily
high if long items used. Avoid this by using a reasonably large but not too
big minimal width instead of determining it automatically from the items width.

Closes #13754.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71301 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775