Source script detection WIP.

On property access, get source script file URLs and line/column numbers
with V8's stack trace API.
Rename some functions for clarity.
Strip line/column numbers from script URLs.
Store and report script URLs.

With placeholder UI.
Merge branch 'master' into source-script-detection
Add note.
Merge branch 'master' into source-script-detection
Add note.
Refactor getAccessCount()
Split up utils to save content script size.

lib/utils got split up into lib/content_script_utils (used/usable by
content scripts and background pages alike) and lib/utils (used by
background pages only).
Merge branch 'source-script-detection'
Update builds.

Not clear why they need updating ...
Add TODOs.