nc_files: add a python based 'filter program' test program

for basic testing of screens that do filtering.
This python version covers us if qt5 is not available.

nc_files/filtertest.py

@@ -0,0 +1,21 @@
+import time
+import sys
+
+for i in range(0,100):
+    try:
+        # simulate calculation time
+        time.sleep(.1)
+
+        # output a line of gcode
+        print >>sys.stdout, 'G0 X1'
+
+        # update progress
+        print >>sys.stderr, 'FILTER_PROGRESS={}'.format(i)
+    except:
+        # This causes an error message
+        print >>sys.stderr, 'Error; But this was only a test'
+        raise SystemExit(1)
+
+
+
+