Don't change mouse position in ui::Slider
Do not change mouse position in PreviewCommand
Remove all "infinite scroll" stuff to avoid changing mouse position (fixes #350)

Changing the mouse position isn't user-friendly in desktop apps, and
has several problems in some platforms (Mac OS X). In this way we've
changed the approach to handle the mouse position when it goes outside
the ui::Editor viewport.
Improve Editor::autoScroll() scroll

Avoid a bad user experience when he/she tries to put back the mouse
position into the editor viewport.