webserver: add support for passing JSON-RPC requests through HTTP GET arguments

In addition to the existing support for sending JSON-RPC requests through HTTP
POST requests to the webserver this commit adds support for JSON-RPC requests
through HTTP GET arguments by passing a full URL-encoded JSON-RPC request in the "request" GET argument.
changed: remove unused IFile callback from GUIWindowFileManager
added: listen to GUI_MSG_UPDATE in GUIWindowFileManager
added: comparison operator to CFileOperationJob
refactor: split off action to string into GetActionString
changed: background file manager operations
added: possibility to cancel file manager operations
cleanup - removed unreferenced functions, moved dangling variable declaration
Merge pull request #1543 from asylumfunk/core.js

[webinterface.default] removed unreferenced functions, moved dangling variable declaration
Merge pull request #1506 from Montellese/jsonrpc_http_get

webserver: add support for passing JSON-RPC requests through HTTP GET arguments
[OSX] cleanup unused variable
Merge pull request #1537 from takoi/stat

[python] extended xbmcvfs with stat
Merge pull request #1541 from cptspiff/bgfileops

changed: background file manager operations