minor updates for other examples

 - Made more extensible by breaking out tick spacing and text generating into separate methods
 - Text now tries harder to avoid overlapping

 - HistogramLUTItem avoids using lookup table if possible
 - GradientEditorItem has a method to ask whether the gradient is trivial (can be applied without the use of a lookup table)
 - ROI, LinearRegionItem, InfiniteLine no longer redraw for every mouse movement

  - Added rate-limited mode to SignalProxy
  - Added basic text justification to LabelItem
  - ViewBox.addItem now has ignoreBounds option, which causes the item to be ignored when autoscaling
  - Added ValueLabel widget
  - Fixed some autoscaling bugs
  - InfiniteLine fix - no hilight if movable=False

- re-enabled OpenGL on windows, added a config option for enabling/disabling OpenGL
- minor bug fixes

Updates to MeshData class (this is still not tested)

 - isocurves
 - array processing through gradientwidget

 (this *should* be a backward-compatible change)

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

  GraphicsObject - corrected bug in viewPos() method
  WidgetGroup - allow bound methods in interfaces
  parametertree - fixed crash when calling remove from context menu

 - initialization error in plotcurveitem
 - performance fix for plotdataitem

Made APIs more complete and consistent.

  - Corrected ImageItem.setRect transformation order  
  - PlotCurveItem uses nkPen for interpreting shadowPen arguments
  - PlotItem and PlotWidget wrap a few more missing methods from ViewBox

documentation update
removed documentation builds from repo.

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)

 - volumetric rendering
 - isosurfaces, mesh rendering
 - basic transformation and parent/child functionality