Support for string constants in Workshop

[ct2034]

• preprocessing step that looks through the bt.xml and all scxmls for string constants
• fixed mapping between strings constants and unique integers (like a enum)
• replace strings with corresponding numbers in bt.xml and scxmls

e.g.
(in bt.xml)

<DetectObject name="DetectSnack" object="snacks0"/>

turns into ...

<DetectObject name="DetectSnack" object="8"/>

and
(in scxml)

<field name="data" expr="_msg.data == 'snacks0'" />

turns into ...

<field name="data" expr="_msg.data == 8" />