The main change is on `ViewBox.getContextMenus`, which now returns an
up-to-date of actions that `GraphicsScene.addParentContextMenus` can use.

Also, `getContextMenus` was given a default implementation in the base
class (falling back on `getMenu` if defined), and some cleanup was done.

  - AxisItem shows 2 tick levels instead of 3
  - Lots of boundingRect and dataBounds caching
    (improves ViewBox auto-range performance, especially with multiple plots)
  - GraphicsScene avoids testing for hover intersections with non-hoverable items
    (much less slowdown when moving mouse over plots)
These are deep changes; need good testing before we release them.

Started new SVG exporter
Merged updates from ACQ4

- GLScatterPlotItem can work with arbitrarily-shaped vertex arrays
- added colorToAlpha function

Bugfixes:
- GraphicsScene emits sigMouseClicked for all clicks
- CanvasItem emits transformation change signal when mirrored
- GLViewWidget.pixelSize correctly handles position specified as array
- SRTTransform3D assumes Zscale=1 when converting from 2D transform

Now detects and switches between PyQt / PySide automatically; no need to distribute different versions of the library anymore.

  - Fixed some floating-point precision issues. (Added a workaround for QTransform.inverted() bug)
  - No longer putting asUnicode inside __builtin__ since this causes problems in some rare circumstances
    (pyshell, lazy import recipe)
  - Minor docstring updates

- Text spacing fix for AxisItem

bugfixes for pyside-specific issues

  - curves enable antialiasing when exporting to image
  - plotitems hide button during export

Fixed a few bugs in AxisItem

Moved GraphicsScene to its own directory, added exportDialog
Removed old export options from PlotItem / ViewBox (will re-enable once they are working again)