* Add a workspace drag surface that blocks and bubble get moved to during a workspace drag.
The surface is translated using translate3d instead of svg's translate attribute so that
the browser does not have to repaint the entire workspace on every mouse move.
This is very similar to the block drag surface.
* Address code review comments
* add back hasClass_ utility removed in #748 and stop using contains since it is not supported in IE
* Split the scrollbar and flyout out into their own SVG elements. They
are siblings of the workpsace SVG. This paves the way to make performance
improvements to workspace dragging.
* Adds message references to message string interpolation, in the form of %{BKY_STRING}.
* Re-adding CONTROLS_IFELSE block using the new syntax, referencing to CONTROL_IF equivalents.
Improve performance of block dragging. This is a backport of the block drag surface from scratch-blocks. At the beginning of a block drag, blocks get moved to a drag surface which then translates using translate3d to avoid repainting the entire svg on every mouse move. At the end of the drag, the blocks are dropped back in the svg in their new position.
from getting selected. Use e.preventDefault instead of adding and removing
classes on mousedown and mouseup. This keeps the browser from having to
potentially recacluate style on mousedown and mouseup events.
css to mark text as unselectable instead. I can't prove this is
significantly faster with numbers since the call to setup the time out
costs ~.1ms and the call when the timer fires is ~.05. This happens on almost
every mouse event though. Plus, it seems to remove a flicker of highlighting when
frames are skipped and keeps scrolling via the scrollbar from highlighting.
