Avoid automatically closing parent frame if a child dialog was closed.

This counterintuitive behaviour could happen when a dialog of a frame
returning false from its ShouldPreventAppExit() was closed: if there were no
other TLWs in the application, then the parent frame could be closed as well,
even if this clearly shouldn't happen.

See #15880.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75633 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Ensure that the help viewer keeps running as long as its main frame is opened.

Do prevent the app from exiting when the help frame is shown in the special
case of the help viewer application where the default behaviour of exiting the
application when only the help window remains open is inappropriate and
results in unexpected program exit if we open and then close any dialog, e.g.
from the help frame toolbar.

Even though the previous commit actually already fixed the problem in the
current code, still apply this fix as well, just in case we can (or could, in
the future) should any dialogs with NULL parent in this program.

Closes #15880.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75634 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
wxWindow::SetWindowVariant() does do something under ports other than wxOSX.

Document that this function adjusts the window font size under non-OS X
systems and explain why it should still be used instead of changing the font
size directly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75635 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775