Fix wxTextEntryDialog::GetValue() when using SetTextValidator().

Associating a validator with the text control in wxTextEntryDialog resulted in
GetValue() always returning an empty string which was completely unexpected.

Fix this by setting the value string manually to avoid interfering with any
possibly used validators.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73774 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Document wxTextEntryDialog::SetTextValidator().

These methods exist since r15925 but were never documented.

See #5420.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73775 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775