[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/Command.cpp

@@ -1252,7 +1252,7 @@ import FreeCAD\n\
 from PySide import QtCore\n\
 import numpy as np\n\
 from matplotlib import pyplot as plt\n\
-plt.figure(1)\n\
+plt.figure(\"" << titleLabel << "\")\n\
 plt.plot(t_coords, membrane, \"k--\")\n\
 plt.plot(t_coords, mb, \"b*-\")\n\
 plt.plot(t_coords, peak, \"r-x\")\n\

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\

src/Mod/Fem/femtaskpanels/task_result_mechanical.py

@@ -504,6 +504,7 @@ class _TaskPanel:
         if len(plt.get_fignums()) > 0:
             plt.close()
         plt.ioff() # disable interactive mode so we have full control when plot is shown
+        plt.figure(res_title)
         plt.hist(res_values, bins=50, alpha=0.5, facecolor="blue")
         plt.xlabel(res_unit)
         plt.title(translate("FEM","Histogram of {}").format(res_title))