Refactor wxDocParentFrame and wxDocMDIParentFrame to share common base class.

Refactor wxDocParentFrame and wxDocMDIParentFrame to share common base class.

Use the same approach as for the child frames: add a base template class which
allows wxDocParentFrame to inherit from wxFrame and wxDocMDIParentFrame from
wxMDIParentFrame while still allowing to reuse the common code.

This reduces code duplication and should make implementing parent AUI document
frame easier as well, see #8945.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64295 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775