Multiselect for slides and components

Done:
- Multi-selecting with shift key pressed + clicking (slides in slide well and components).
- Mass copy/paste.
- Mass removal.
- Mass class assignment.
- A lot of refactoring (see commit description for details). I've inserted github comments into commits at some places to help you understand the changes, but diffs on github still look horrible; I had a better luck reviewing changes in my IDE. Really sorry for that.

Known issues:
- No multiselect in overview mode yet.
- No multidragging yet.
- No rectangular selection yet, just a Shift+click.
- Shift+click should be re-programmed to work as a range-selector instead of pick-selector. Cntrl/Cmd keys should work as pick selectors instead. But for now, I think it's okay.
- It looks like etch doesn't really fits multiselect scenario in terms of assigning text styles. Standard editor bar (global) would probably do a better job. Here's example: if you select two textboxes and want to change their font.