Use helper GetWeek{Start,End}() functions instead of repeating tests for
wxCAL_MONDAY_FIRST over and over again.
Also replace some occurrences of GetWindowStyle() with shorter and more clear
HasFlag().
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65902 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775