SWTException: Graphic is disposed when open ContextDialog a second time

Since the HOP-4469 fix, when you close a ContextDialog, the Item.dispose() method disposes of the image that was keep in a cache and the second time you open it you get an exception.

Before the fix this was not a problem because a new image is recreated each time without using the cache.

I think we should not dispose the images in the context dialog.

org.eclipse.swt.SWTException: Graphic is disposed
    at org.eclipse.swt.SWT.error(SWT.java:4918)
    at org.eclipse.swt.SWT.error(SWT.java:4833)
    at org.eclipse.swt.SWT.error(SWT.java:4804)
    at org.eclipse.swt.graphics.Image.getBounds(Image.java:1245)
    at org.apache.hop.ui.core.dialog.ContextDialog.onPaint(ContextDialog.java:871)
    at org.apache.hop.ui.core.dialog.ContextDialog.lambda$open$8(ContextDialog.java:495)
    at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:89)
    at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4251)
    at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1066)
    at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1090)
    at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1075)
    at org.eclipse.swt.widgets.Composite.WM_PAINT(Composite.java:1536)
    at org.eclipse.swt.widgets.Control.windowProc(Control.java:4802)
    at org.eclipse.swt.widgets.Canvas.windowProc(Canvas.java:340)
    at org.eclipse.swt.widgets.Display.windowProc(Display.java:5036)
    at org.eclipse.swt.internal.win32.OS.DispatchMessage(Native Method)
    at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3643)
    at org.apache.hop.ui.core.dialog.ContextDialog.open(ContextDialog.java:539)
    at org.apache.hop.ui.hopgui.context.GuiContextUtil.handleActionSelection(GuiContextUtil.java:167)
    at org.apache.hop.ui.hopgui.file.pipeline.HopGuiPipelineGraph.showActionDialog(HopGuiPipelineGraph.java:1281)
    at org.apache.hop.ui.hopgui.file.pipeline.HopGuiPipelineGraph.mouseUp(HopGuiPipelineGraph.java:1084)
    at org.eclipse.swt.widgets.TypedListener.handleEvent(TypedListener.java:224)
    at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:89)
    at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4251)
    at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1066)
    at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:4068)
    at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3645)
    at org.apache.hop.ui.hopgui.HopGui.open(HopGui.java:415)
    at org.apache.hop.ui.hopgui.HopGui.main(HopGui.java:307)