xxf:r() must support indexes

[ebruchez]

Goal: support multiple alerts.

• local branch 2013-07-01-update-xxf-r-to-support-indexes
• alert format for later: xxf:r('control-1.alert.2')
• FB/FR must support using xxf:r()

[ebruchez]

Needed for #1342.

[ebruchez]

• Q: Index from 0 or from 1?
  • → let's pick 0
• implement search
• implement PathMap
• unit tests for xxf:r()
  • Scala tests of path evaluation
  • XPath dependencies test

[ebruchez]

After discussion:

• change the syntax to control-1.alert[2]

This makes things more consistent with the XPath and JavaScript syntaxes for indexes.

[ebruchez]

For #1342, we moved to using nested <fr:param> within <xf:label>, etc. But at runtime, we convert that to using xxf:r(). Right now, for LHH, we use control-name.label, etc., but we will need the .2 or [2] syntax for supporting alerts. So this goes hand in hand with supporting dynamic alert messages with #3628.

[ebruchez]

Since support for dynamic alert messages has moved to #3628, we can postpone updating the syntax as well.