Google Drive and Dropbox resolvers

Changes necessary in Tomahawk to implement Google Drive and Dropbox resolvers.

Features : 
* Resolvers can register to signals of their config.ui dialog.

Usage : 
``	fields: [{
					name: "associateButton",
					widget: "associateButton",
					property: "text",
					connections : [ { 
							  signal: "clicked()", 
							  javascriptCallback: "resolver.associateClicked();" 
							}]                
			}]``