Accept lowercase values for Action.placementPriority

The clients support "LOW", "HIGH", and "NORMAL". Since lower-case
keywords are used throughout the tabris.js API, accept these strings
in lower case and adjust the case internally.

For the sake of consistency, don't make the properties case-insensitive,
i.e. *only* lower case is accepted.

Change-Id: I5755062103bc4643bd78fffabc71af4bb9e8c3a8