Multidragging components

This pull request resolves #193

Other changes:
1. I've returned ability to Shift+click components. It should work just the same as Cmd/Ctrl+click.
2. Created css class for slide placeholder and assigned it the same styles as slide snapshot has (including margin, border and border-radius). This makes it more saxy and less annoying due to margin reset when you move slides around.
3. Fixed wrong behavior of Shift+click multiselect for slides, where old selection is not reset when you run new Shift+click. Now, you can do following stuff:

(given you have 10 slides and the 3rd is active)
1. Shift+click 5th slide (3-5 will be selected).
2. Shift+click 1st slide (1-3 will be selected, 4 and 5 not)
3. Ctrl+click 5th slide (1-3,5 will selected)
5. Shift+click 7th slide (3-7 will be selected, including 4th)


Note the 5th behavior. It's not how it works in file browser, where you'd have [1-3,5-7] selected. Implementing such system would take much more time and I don't think it's reasonable. Besides, even Keynote has it the way it's done in this pull request.