Expression support

- Added Context::getColumns to return a vector of column names.
  This is to help a shift toward using the Column objects to assist
  in the parsing/validation of data entry/modifications.
- Added Column::modifiable to delegate an attributes readable/writeable
  state.  This means the columns will be in charge of their own
  mutability, which will simplify and generalize Command::modify_task.

src/columns/ColID.cpp

@@ -38,10 +38,11 @@ extern Context context;
 ////////////////////////////////////////////////////////////////////////////////
 ColumnID::ColumnID ()
 {
-  _name  = "id";
-  _type  = "number";
-  _style = "number";
-  _label = STRING_COLUMN_LABEL_ID;
+  _name       = "id";
+  _type       = "number";
+  _style      = "number";
+  _label      = STRING_COLUMN_LABEL_ID;
+  _modifiable = false;
 
   _styles.push_back ("number");