ScriptInfo: use File instead of String

The class still needs further augmentation to support
arbitrary Reader + String pseudopaths as well, though.
ScriptInfo: tweak javadoc
ScriptInfo: fix position of instance field
ScriptInfo: make helper method private
ScriptInfo: remove unused method
ScriptInfo: allow inputs to be reparsed

This is useful if the source file has changed
(e.g., if it was edited in the Script Editor).
WIP