Add wxEventLoopBase::DoRun().

Call it from public Run() after setting the loop as active and resetting
m_shouldExit flag.

No real changes, just cut down on the code duplication among the ports and
make it easier to implement the upcoming changes. see #10258.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74333 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775