Debugger: Delay matplot process initialization

Reduces costs in case the feature is not requested.

Change-Id: I66da6f256baaec6ef9d40bc135942551d58e83da
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

share/qtcreator/debugger/dumper.py

@@ -91,7 +91,14 @@ except:
 if hasSubprocess and hasPlot:
     matplotFigure = {}
     matplotCount = 0
+    matplotProc = None
+    devNull = None
 
+    def matplotInit():
+        global matplotProc
+        global devNull
+
+        if matplotProc is None:
             devNull = open(os.devnull)
             # FIXME: That might not be the one we want.
             pythonExecutable = sys.executable
@@ -109,6 +116,8 @@ if hasSubprocess and hasPlot:
         global matplotFigure
         global matplotCount
 
+        matplotInit()
+
         def s(line):
             matplotProc.stdin.write(line.encode("latin1"))
             matplotProc.stdin.write(b"\n")
@@ -136,6 +145,8 @@ if hasSubprocess and hasPlot:
         matplotProc.stdin.flush()
 
     def matplotQuit():
+        global matplotProc
+        if not matplotProc is None:
             matplotProc.stdin.write(b"exit")
             matplotProc.kill()
             devNull.close()