Added isValidSymbolName, fixed null pointer error in encodeString().

Change-Id: I67109eb5f4eb463b4a1d277a24c16cb18c20bb6f
EquationEditor drag/drop now checks if variable name is a valid symbol
name before wrapping in a call to get().
Fixed null pointer error in drag/drop.
Improved interface.

Change-Id: I442c1c492336d4226e980cb36482b53e1f3ff867