Get Books: Improve searching by removing punctuation

Get Books: Improve searching by removing punctuation from title/authors
before matching.

Merge branch 'master' of https://github.com/user-none/calibre
Make the punctuation removal faster and more robust
Comments editor: Disable insert link until focused

Comments editor: The Insert Link button has no effect until the user
clicks inside the comments box, therefore disable it until it is ready,
to prevent confusion. Fixes #1208073 [Insert Link not working](https://bugs.launchpad.net/calibre/+bug/1208073)
pep8
Make it clear that disabling plugins is prevented by policy
pep8
EPUB Input: Use raster cover metadata

EPUB Input: If the EPUB file identifies an actual cover image in
addition to the titlepage html file, use the cover image instead of
rendering the titlepage. This is faster and has the advantage that an
EPUB to EPUB conversion preserves internal cover structure.
Support adding images into the comments field

Allow adding images into the comments field, by clicking on the insert
link button in the comments editor in the edit metadata dialog.

When generating a metadata jacket during book polishing or conversion,
embed any images referenced in the comments.
Switch to reading plugin index from plugins mirror
Delete Activity entry for synced shelves

When deleting shelves that have been synced, the Activity entry for the
shelf was not being deleted. This left a tile for the shelf on the home
screen of the Glo and AuraHD. Fixes #1208159 [Delete Activity entry for synced shelves on Kobo devices](https://bugs.launchpad.net/calibre/+bug/1208159)