Fixes #369 - Duplicated prefs in browser.storage.local

This now migrates all the file records stored in browser.storage.local prefixed with "file:" (pre-SDK ABPUI) and "abp:file:" (ABP on sdk 0.5.0) to what we currently use ("ewe:file:). It was partly confirmed by Thomas to help (at least "file:" -> "abp:file:"). Also adding the test to make sure.

Note we simulate the old entries from pre-sdk abpui in the test (so it's not 100% same case).

Also we back-porting the changes to "0.6.0" (likely to become "0.6.1") in the branch.

Closes #369 (closed)