VirtualRasterBuilder: added removeVirtualBands(), addVirtualBand(), insertVirtualBand(), addFilesAsStack(), addFilesAsMosaic(), improvements to saveVRT()

fixed labelSummary text update

UI fixes

changed DatumView.sizeHint()
SpatialTemporalVisualization uses RefreshTimer to handle very frequent refresh requests

added niceNumberString(number) to get human readable number strings

- new context menu actions, e.g. set stretch to current area
- stretchToExtent(self, spatialExtent, stretchType='linear_minmax', **stretchArgs)
  to support different stretch types
- lazy loading of new data sources,

improved QGIS interaction
several UI issues and smaler bugs fixed

widget "under construction" will not be shown to the user

MapCanvases get registered at responsible Controllers (STV, MV and DV)
utils.saveTransform(geom, crs1, crs2) takes care on invalid coordinate transformations

refactoring