gui: don't scroll experiment editor on modified wheel (closes #303)

If you try to target e.g. spinbox with a ctrl- or shift-wheel to change the value
and you don't aim well, the tree widget will scroll. We already have tree
scroll on unmodified wheel. Let's have the modified wheel events exclusively
target the smaller widgets.

artiq/gui/experiments.py

@@ -18,6 +18,15 @@ logger = logging.getLogger(__name__)
 # 2. file:<class name>@<file name>
 
 
+class _WheelFilter(QtCore.QObject):
+    def eventFilter(self, obj, event):
+        if (event.type() == QtCore.QEvent.Wheel and
+                event.modifiers() != QtCore.Qt.NoModifier):
+            event.ignore()
+            return True
+        return False
+
+
 class _ArgumentEditor(QtWidgets.QTreeWidget):
     def __init__(self, manager, dock, expurl):
         self.manager = manager
@@ -38,6 +47,8 @@ class _ArgumentEditor(QtWidgets.QTreeWidget):
         self.setHorizontalScrollMode(QtWidgets.QAbstractItemView.ScrollPerPixel)
         self.setVerticalScrollMode(QtWidgets.QAbstractItemView.ScrollPerPixel)
 
+        self.viewport().installEventFilter(_WheelFilter(self.viewport()))
+
         self._groups = dict()
         self._arg_to_entry_widgetitem = dict()