Draft: Finish command if user deselected Chained Mode

As the title says - currently, if user deselects Chained Mode, they
can't exit it without using ESC key for example, as the old behavior
of ContinueMode is kept under it.

So, this patch finishes the command if user has deselected it, at
the same time finishing previous chain.

src/Mod/Draft/DraftGui.py

@@ -940,6 +940,10 @@ class DraftToolBar:
         params.set_param("ChainedMode", bool(val))
         self.chainedMode = bool(val)
         self.continueCmd.setEnabled(not val)
+        if val == False:
+            # If user has deselected the checkbox, reactive the command
+            # which will result in closing it
+            FreeCAD.activeDraftCommand.Activated()
 
     # val=-1 is used to temporarily switch to relativeMode and disable the checkbox.
     # val=-2 is used to switch back.

src/Mod/Draft/draftguitools/gui_dimensions.py

@@ -74,7 +74,7 @@ class Dimension(gui_base_original.Creator):
     def __init__(self):
         super().__init__()
         self.max = 2
-        self.chain = None
+        self.chain = None # Last chain's leg in ChainMode
         self.contMode = None
         self.dir = None
         self.featureName = "Dimension"