Add drag drop to mantid QT Widgets

This adds drag drop functionality to all instances of
* MWRunfiles
* WorkspaceSelector
* DataSelector
This will be on by default for all instances of the widgets, but can be disabled by calling
 widget->acceptsDrops(false)

MWRunFiles will accept dropped files, and text, but NOT workspaces

WorkspaceSelector will select dropped workspaces if that selection is valid for the control

DataSelector will change to the MWRunfiles or WorkspaceSelector based on the drop and act as the control above

To Test:
1. Open up mantidplot, load a workspace or two, and have some external files ready
1. Open the SANS ISIS interface and into one of the 6 input file boxes try dropping workspaces(should not work) or files, or text from outside mantidplot (should work)
1. Open ConvertToEnergy, select IRIS and process a couple of IRIS files from autodata (just load the file and hit process.
1. Open Indirect DataAnalysis and go to the Fury tab
1. Click on the file dopdown and select workspace you should have entries for the reduced workspaces you have just created.
1. Have fun dropping owkrspaces or external files

re #8978
Merge branch 'feature/8978_Extend_dragdrop' into develop