Remove -Wshadow warnings

[rbuj]

mate-calc.c:186:21: warning: declaration of ‘window’ shadows a global declaration [-Wshadow]
  186 | quit_cb(MathWindow *window)
      |         ~~~~~~~~~~~~^~~~~~
--
math-buttons.c:396:22: warning: declaration of ‘label’ shadows a previous local [-Wshadow]
  396 |         const gchar *label;
      |                      ^~~~~
--
math-converter.c:148:29: warning: declaration of ‘iter’ shadows a previous local [-Wshadow]
  148 |                 GtkTreeIter iter;
      |                             ^~~~
--
math-variable-popup.c:207:23: warning: declaration of ‘value’ shadows a parameter [-Wshadow]
  207 |             MPNumber *value;
      |                       ^~~~~
--
mp-serializer.c:162:22: warning: declaration of ‘digits’ shadows a previous local [-Wshadow]
  162 |         const gchar *digits[] = {"₀", "₁", "₂", "₃", "₄", "₅", "₆", "₇", "₈", "₉"};
      |                      ^~~~~~