Allow creating wxSingleInstanceChecker with default name.

Allow creating wxSingleInstanceChecker with default name.

This makes it easier to use in common cases: there is no need to come up with
a unique name for the checker any more as sufficiently unique combination of
wxApp::GetAppName() and wxGetUserId() is used if no name was explicitly given.

This is done by calling the new CreateDefault() on demand from
IsAnotherRunning() instead of simply creating the checker with the default
name in the default ctor for compatibility (you had to call Create() after
using the default ctor before and it can only be called once) and because
wxTheApp might not exist yet when wxSingleInstanceChecker is created.

Closes #11166.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61945 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775