Fix bug with mouse wheel scrolling wxStyledTextCtrl in long running programs.

Fix bug with mouse wheel scrolling wxStyledTextCtrl in long running programs.

In long running programs the wxEvent time stamp could wrap around resulting in
all mouse wheel events being ignored in wxStyledTextCtrl as the comparison of
the (positive) time until which all the subsequent events were supposed to be
blocked and the (now negative) current event time stamp would be always false.

Fix this by using wxStopWatch::TimeInMicro() to avoid wraparound instead of
wxEvent::GetTimestamp().

Also rename the variable to have a more clear name as the original code wasn't
easy to understand.

Closes #9057.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70500 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775