Refs #10357.  Improved loading.

On branch feature/10357_eventfilter_gui_load
  modified:   Mantid/scripts/FilterEvents/MainWindow.ui
  modified:   Mantid/scripts/FilterEvents/Ui_MainWindow.py
  modified:   Mantid/scripts/FilterEvents/eventFilterGUI.py
Including
(1) Changed the label to load file;
(2) Enabled to load by run number, nexus file name or
instrument_runnumber;
(3) Name of output workspace is made consistent as
InstrumentShortName_RunNumber_event

  modified:   Mantid/scripts/FilterEvents.py
An unnecessary output is removed
Merge branch 'feature/10357_eventfilter_gui_load' into develop