With this change we are removing specific code to scroll the Editor widget.
Now if we use Editor::setEditorScroll() to scroll it should work as
View::setViewScroll(). In this way we remove the ugly "blitValidRegion"
parameter from setEditorScroll().
Changes:
* Removed Widget::scrollRegion() because each widget must control
valid/invalid regions after a ui::move_region() in a very specific way
(see View::setViewScroll() or Window::moveWindow)
* Invalidate the whole widget on Widget::setBoundsQuietly()
* Fixed problems blitting invalid regions/not yet updated/painted: using
the new ui::Manager::m_invalidRegion.
* Added View::onSetViewScroll() and View::onScrollRegion()
* Added FileListView to avoid moving the thumbnail region when we scroll
