Changes that include connecting line data tips, connecting line selection, and layer management.
Reinstated dblclick handler in .click_background
Changes made so that we make use of layerManager
Improved icon
* Fixes based on code review notes

connecting_line_adornment.js [ Replace string with LayerNames constant. ]
dot_plot_view.js [ In creation of adornments, add shadingLayerName ]
layer_manager.js [ No longer descends from SC.Object. in moveElementFromTo, if fromLayer and ToLayer are the same, bail immediately. Use DG.logError in testValidity. ]
line_data_tip.js [ Use localizable, parameterized string in getDataTipText.
plot_adornment.js [ In destroy, don’t pretend to know which layer elements are in. Instead ask layerManager to remove them. ‘layer’ is no longer cacheable. ]
plot_background_view.js [ Added intervalShading layer for use by interval adornments. Cleaned up initLayerManager. ]
plot_utilities.js [ Added comment to doHideRemoveAnimation. ]
plot_view.js [ In drawData, removed redundant call to removeElement. In updateSelection, removed unused local variable. In hideDataTip, check for existence of dataTip. In plottedCountChanged, replace layer name string with constant from LayerNames. ]
plotted_average_adornment.js [ Adornments that involve shading, now place the rectangles used for shading in the kShadingLayer layer. ]
point_data_tip.js [ in getDataTipText, protect against possibility that cases don’t yet exist. Remove hide method as unnecessary override. ]
raphael_base.js [ Changes to accommodate LayerManager as no longer SC.Object. Remove initLayerManager call from didCreateLayer as unnecessary given that we use lazy instantiation. ]
raphael_layer.js [ Fix bug in contains method. ]
scatter_plot_view.js [ In updateSelection, call sc_super first instead of last ]
strings.js [ Added DG.DataTip.connectingLine ]
tool_tip.js [ ‘layer’ property now not cacheable. ]