move guts of TimeoutAlert and NotifyAlert to a private class
move guts of TimeoutAlert and NotifyAlert to a private class

Create a private class for the button, timeout and response.  Use the
private class as parent for both NotifyAlert and TimeoutAlert.

Extends the spurious response fix (cfeabdc) to NotifyAlert.

Also use g_timeout_add in preference to g_timeout_add_seconds, because
the latter is less precise in implementation.

Tested by changing the Register alert temporarily.

Daksh/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,Daksh/sugar-toolkit-gtk3,i5o/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3,quozl/sugar-toolkit-gtk3