Refactor listbox event sending code to avoid duplication.

Refactor listbox event sending code to avoid duplication.

wxMSW wxListBox implementation contained the same code as the private
LBSendEvent() function in lboxcmn.cpp, so make this function a (protected)
member of wxListBoxBase and reuse it instead.

Also change its and CalcAndSendEvent() return type to bool to be able to
return whether the event was processed or not.

As the result of this refactoring, the "is selected" flag is now set correctly
for the selection events under MSW (it was always off before).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64496 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775