0025907: Optimization of testdiff command

- Work with strings optimized in Tcl procedures used in testdiff command
- CPU and memory differences output of testdiff command improved to give relative change in percents
- Cumulative CPU and memory differences are output for test grids
- In HTML output of testdiff command, deviations of memory and CPU measurements greater than 5% are colored (red or green)
- Search of image files in testdiff command corrected to avoid wrong attribution of image file to issues starting with the same first letters; images must start with the test case name, optionally followed by underscore or dash and arbitrary text
- Image_Diff tool optimized for the case if images are exactly the same
- Perf_Meter class output corrected, destructor made non-virtual
- DRAW command diffimage optimized to not save diff files if there is no difference
- Tests User Guide updated according to these changes and actual state

src/ViewerTest/ViewerTest_ViewerCommands.cxx

@@ -5281,7 +5281,7 @@ static int VDiffImage (Draw_Interpretor& theDI, Standard_Integer theArgNb, const
   theDI << aDiffColorsNb << "\n";
 
   // save image of difference
-  if (aDiffImagePath != NULL)
+  if (aDiffColorsNb >0 && aDiffImagePath != NULL)
   {
     aComparer.SaveDiffImage (aDiffImagePath);
   }