Fix issue 309: Program crashes immediately after setting ScreenScale to 1

Avoid double destroy_bitmap() call.
Stop propagation of keyboard messages if a child use it

This fix a new bug with the new EditorView's scroll-bars. As 1,2,3,4,5,6
keys change the zoom, they will change the viewport size, and scroll-bars
will be removed/added. We cannot continue iterating the View::m_children
collection after m_children is modified (scroll-bars are removed/added).