[Tizen] Add signature data and parser logic

SignatureFile: according to widget spec, widget should includes
signature file in the zip. SignatureFile is used to label signature
files in the widget. There are two types of signature files: author
and distributor signature.

SignatueData: it is used to store values parsing from signature.xml.
The data will be used by signature verifier to validate digital
signatures.
Don't force the usage of Chromium's bundled libxml. Include `libxml/parser.h` etc instead. Only libxml_utils should be specified with a full path because it is built regardless of whether `use_system_libxml` is set.