Only create button bar on conceptdiagram start

Note that the Start button is provisional in nature. Hopefully there
will be a point in the future when I'll have sufficiently wrapped my
head around GWT/JavaScript/etc initialisation that we can do away
with it altogether. All I know is that without the natural delay
introduced by having to press the button, you can trigger some sort
of weird race where the JavaScript code sometimes doesn't work at all.
WIP: also save/load curve position
WIP: Automatically load diagram on project start
WIP: Load and save entire diagrams

Still some flakiness involved, namely that when you have multiple curves
of the same name, they get reloaded in the same location an not their
original.

Will have to dig into the causes of this
Upgrade to Guava 16