[FEM] fix issue that all plot windows had the same name

- they all had the name "figure 1"
- thus make the name unique and this way also enable to have several plots open displaying different result types (for the line filter)

src/Mod/Fem/Gui/TaskPostBoxes.cpp

@@ -739,7 +739,7 @@ from PySide import QtCore\n\
 import numpy as np\n\
 from matplotlib import pyplot as plt\n\
 plt.ioff()\n\
-plt.figure(1)\n\
+plt.figure(title)\n\
 plt.plot(x, y)\n\
 plt.xlabel(\"" << xlabel.toStdString() << "\")\n\
 plt.ylabel(title)\n\