Allow Expressions to be set for LABEL properties using MapScript (#6904)

This pull request allows the new functionality in #6884 of using an expression in a LABEL PRIORITY in MapScript. 
It takes a similar approach to setting attribute bindings for properties, which requires using the appropriate attribute constant such as `MS_LABEL_BINDING_PRIORITY`:

```python
label = mapscript.labelObj()
label .setExpressionBinding(mapscript.MS_LABEL_BINDING_PRIORITY, "[MY_ATTRIBUTE] * 2")
```

This is a little clunky but is at least consistent with setting attribute bindings:

```python
label = mapscript.labelObj()
label.setBinding(mapscript.MS_LABEL_BINDING_COLOR, "NEW_BINDING")
```

While testing this I noticed that the `writeLabel` function used to write Mapfile objects ignored expressions. This seems an oversight also for the `LABEL` `SIZE` expression. If this approach seems appropriate I can use this for other expression bindings.

Something to note is that a property could have an attribute binding **and** an expression binding set. In this case the attribute binding currently takes precedence when calculating a shape value, so it also takes precedence when writing to a Mapfile. 

In theory setting an expression could also set the attribute binding to NULL. This is not currently the case for MapScript or when reading a Mapfile twice.

mapfile.c

@@ -2022,6 +2022,8 @@ static void writeLabel(FILE *stream, int indent, labelObj *label)
 
   if(label->numbindings > 0 && label->bindings[MS_LABEL_BINDING_PRIORITY].item)
     writeAttributeBinding(stream, indent, "PRIORITY", &(label->bindings[MS_LABEL_BINDING_PRIORITY]));
+  else if (label->nexprbindings > 0 && label->exprBindings[MS_LABEL_BINDING_PRIORITY].string)
+    writeExpression(stream, indent, "PRIORITY", &(label->exprBindings[MS_LABEL_BINDING_PRIORITY]));
   else writeNumber(stream, indent, "PRIORITY", MS_DEFAULT_LABEL_PRIORITY, label->priority);
 
   writeNumber(stream, indent, "REPEATDISTANCE", 0, label->repeatdistance);

mapscript/python/tests/cases/label_test.py

@@ -38,6 +38,17 @@ def testLabelBinding(self):
         new_label.setBinding(mapscript.MS_LABEL_BINDING_COLOR, "NEW_BINDING")
         assert (new_label.getBinding(mapscript.MS_LABEL_BINDING_COLOR) == "NEW_BINDING")
 
+    def testLabelExpressionBinding(self):
+        """expression binding can be set and get"""
+        new_label = mapscript.labelObj()
+        assert (not new_label.getExpressionBinding(mapscript.MS_LABEL_BINDING_PRIORITY))
+        new_label.setExpressionBinding(mapscript.MS_LABEL_BINDING_PRIORITY, "[MY_ATTRIBUTE] * 2")
+        assert (new_label.getExpressionBinding(mapscript.MS_LABEL_BINDING_PRIORITY) == "([MY_ATTRIBUTE] * 2)")
+
+        exp = 'LABEL\n  SIZE 10\n  OFFSET 0 0\n  POSITION CC\n  ' \
+              'PRIORITY ([MY_ATTRIBUTE] * 2)\n  SHADOWSIZE 1 1\nEND # LABEL\n'
+        assert (new_label.convertToString() == exp)
+
 
 class LabelCacheMemberTestCase(MapTestCase):
 

mapscript/swiginc/label.i

@@ -112,7 +112,52 @@
     return MS_SUCCESS;
   }
 
-  /// Set the label expression.
+  /// Remove an expression binding for a specified label property.
+  int removeExpressionBinding(int binding) 
+  {
+    if(binding < 0 || binding >= MS_LABEL_BINDING_LENGTH) return MS_FAILURE;
+
+    if(self->exprBindings[binding].string) {
+      msFreeExpression(&self->exprBindings[binding]);
+      self->nexprbindings--;
+    }
+
+    return MS_SUCCESS;
+  }
+
+  /// Get the expression binding for a specified label property. Returns NULL if there is no binding for this property.
+  %newobject getExpressionBinding;
+  const char *getExpressionBinding(int binding) 
+  {
+    if(binding < 0 || binding >= MS_LABEL_BINDING_LENGTH) return NULL;
+
+    return msGetExpressionString(&(self->exprBindings[binding]));
+  }
+
+  /// Set the expression binding for a specified label property. Binding constants look like this: ``MS_LABEL_BINDING_[attribute name]``
+  /// Expressions are automatically wrapped in brackets, so do not need to be added to the input string
+  /// >>> new_label.setExpressionBinding(MS_LABEL_BINDING_PRIORITY, "[priority] * 2")
+  int setExpressionBinding(int binding, const char *text) 
+  {
+    if (!text || strlen(text) == 0) {
+      return MS_FAILURE;
+    }
+
+    if(binding < 0 || binding >= MS_LABEL_BINDING_LENGTH) return MS_FAILURE;
+
+    if(self->exprBindings[binding].string) {
+      msFreeExpression(&self->exprBindings[binding]);
+      self->nexprbindings--;
+    }
+
+    self->exprBindings[binding].string = msStrdup(text);
+    self->exprBindings[binding].type = MS_EXPRESSION;
+    self->nexprbindings++;
+
+    return MS_SUCCESS;
+  }
+
+  /// Set the label expression property
   int setExpression(char *expression) 
   {
     if (!expression || strlen(expression) == 0) {