Add a virtual RimMultiPlot::handleDroppedObjects so RiuMultiPlotBook routes drop
events to any multi plot type instead of only RimSummaryMultiPlot. Implement the
handler in RimHistogramMultiPlot: each dropped ensemble summary vector, ensemble
parameter or file set ensemble creates its own new histogram subplot, following
the behavior of the summary multi plot. Extract RimHistogramPlot::isDroppableObject
so unsupported dropped objects do not create empty subplots.