correct selection and multiple items dragging

app/components/commander/index.js

app/components/viewer/document/index.hbs

app/components/viewer/document/index.js

app/components/viewer/index.hbs

app/components/viewer/index.js

app/components/viewer/page.hbs

app/components/viewer/pages.hbs

app/components/viewer/thumbnail/index.hbs

app/components/viewer/thumbnail/index.js

app/components/viewer/thumbnails.hbs

app/modifiers/draggable.js

app/modifiers/scroll_into_view.js

app/modifiers/ui_select.js

app/utils/rectangle.js

tests/unit/rectangle-test.js