ENH: Add qSlicerScriptedFileDialog

* Add qSlicerFileDialog::description
* Add input dialog to select reader when multiple readers accept the same mime data


How to create a FileDialog in python?

+class XYZFileDialog:
+  def __init__(self, parent):
+    self.parent = parent
+    parent.fileType = 'XYZ'
+    parent.description = 'XYZ'
+    parent.action = slicer.qSlicerFileDialog.Read
+  def isMimeDataAccepted(self):
+    accept = self.parent.mimeData().hasFormat("text/uri-list")
+    self.parent.acceptMimeData(accept)
+  def dropEvent(self):
+    self.parent.dropEvent().accept()
+  def execDialog(self):
+    print 'exec'

How to order the list (make the module reader by default):
Make the module depend on "Data" -> instantiated after "Data" module
   def __init__(self, parent):
+    parent.dependencies = ["Data"]

Issue #3081

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21951 3bd1e089-480b-0410-8dfb-8563597acbee