Use wxBufferedPaintDC to implement double buffering in wxPG.

This also fixes drawing the PG when buffer is not available and there is necessary do draw directly on window DC.
wxPropertyGrid::DrawItems method is simplified and wxPropertyGrid::DoDrawItems method is reimplemented since its 3-rd argument (isBuffered) is unneeded anymore.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78266 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Create wxPropertyGridManager with default size in the propgrid sample

Since r78150 wxPropertyGridManager can be created with default size with no issues.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78267 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775