Add lots of buttons; Implement "delete" mode

In order to motivate myself to implement the remaining features, I added buttons
for them that won't do anything yet. I styled them with the table style, just
like the buttons in the controls view. I also made all the buttons unselectable,
which I should have done a while ago. Finally, I implemented the delete mode.

Clicking on the DEL button switches you into delete mode, where you click on
points to delete them rather than adding new points. You can also delete the
first point -- the new first point will become the old second point, and it will
become red as expected. Clicking on the DEL button again returns you to the
regular mode (it remains bright red while you are in delete mode -- I think it
is better than having the text change to ADD or something similar, because it
could be confusing unless it is very obvious when you are in delete mode). I
refactored drawing.js quite a bit in this process, noticing several repeated
series of function calls.