Use styleSheetAdded and styleSheetRemoved, replaces getAllStylesheets

Fix for #6830 
* Add `styleSheetAdded` and `styleSheetRemoved` event handling to inspector protocol
* Fallback to `getAllStylesheets` for Chrome < 34

Fix for #6912 
* Use `styleSheetAdded` to keep `styleSheetId` values up to date to fix "No style sheet with given id found, code: -32000" when pushing change events from Brackets to Chrome