Add wxProgressDialog::Was{Cancelled,Skipped}() convenience methods.

Add wxProgressDialog::Was{Cancelled,Skipped}() convenience methods.

Although the information about "Cancel" and "Skip" buttons presses is returned
from Update(), sometimes it may be more convenient to ask the dialog about
whether it was cancelled or skipped instead of storing it in the program
itself.

Add the methods which allow to check for this.

Closes #10903.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64057 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775