Stylization: Fix handling of default values

[louisewatson]

In SE_ExpressionBase, there are structures to manage all of the basic data types used in stylization. These structures are not managing default values properly.

[louisewatson]

The fix is to store the appropriate default value in each of the data structures contained in SE_ExpressionBase.h. These are SE_Double, SE_String, SE_Boolean, etc. Now when the structure's evaluate function is called, if the expression does not yield a valid value, the default value is used.

Note that the changes to SE_String were more extensive in order to insure that memory was used properly and efficiently. To get the current value, getValue() should be called.