Use automatically generated ID values for wxPG sub-controls.

Currently in wxPG there are used fixed "magic" ID values to identify sub-controls and this can lead to side effects if these values overlap with ID values assigned to another controls in the application (like e.g. menu items).

Closes #13634.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76882 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775