improve GEE Time Series Explorer app

[Andreas Janz]

• fix #955

• use QgsDateTimeEdit widget with clear button to reset to original start end date of the collection:

  

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• clear RGB Vis. selection after user picks bands manually

  

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• implement copy data table button:

  

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[Andreas Rabe]

Also, add Landsat 9 collection.

[Andreas Rabe]

Done, added Landsat 9:

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