Merge pull request #8 from dvinc/master

VIBes viewer: new figure windows to foreground, fixed graphic glitches with axes
Simplified the C++ API even more, +passing const ref to string instead of string copy
C++ API: remember last selected figure with figure("name") when calling
commands.
VIBes viewer: added the "view" action, which sets the current view box.
It also supports automatic zoom.