Search through all properties in wxPG::GetPropertyByLabel.

Function should search for given label through all properties. This is consistent with searching by name (in GetPropertyByName) which is not limited to the subset of properties.

Closes #15251.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76926 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775