background link and image checks and comment detection

Migrated more functioanality over from the Test Tool Hub.

Link and Image checking happens in background as you test and auto creates bug notes.

To differentiate from any of the many other link checking extensions and tools and fit in with the 'you focus on testing, this tool observes' ethos.

• migrated html comment detection from Test Tool Hub
• added more options to configure event checking
• removed option setting from popup - need to use the options views now
• background and manual trigger broken link check - based on link check in test tool hub
• background and manual trigger broken image check
• status rendering of background info - monitored domain (in image alt), and link and image checking queue length