Refactor hotkeys & add tool icon tooltips.

- [x] Refactors hotkeys into respective tool files.
- [x] Removed duplicate / unused hotkeys.
- [x] Adds description tooltips to each tool icon.
- ~~Refactor Menus and Tools to propagate from class files?~~ (nah - https://github.com/artursapek/mondrian/pull/33#discussion_r9332400)


Will close #20.

(The reason I bundled these together is that tooltips would have depended on some of the new hotkey things. Since we are not refactoring them at this time, however, that is not the case.)