add ui for inline verification

plainly copied from inline version of decrypt and adjusted to
verification of a message.
add verification script to main script

verification code needs to be in concatenated concat script, so it will be
available to load.
add needed files to chrome manifest

without these entries, chrome won’t load the files.
add logic to verify message signatures

once triggered, messages will now be verified and the result of the
verification is sent back to the view to present it.
hook verification code into mailvelope code

PGP signatures will now be detected and a verification button will be shown
to let the user verify the signature.
refactor frames to not use JS for handling element sizing

all sizing is now done in CSS. For that, the actual element that will be
overlayd by the frame, has been wrapped within a wrapper element, that
can be styled individually. CSS is then used to achieve the same effect
(automatically sizing the element to fit the original pgp element).
add decent icon for verify action

looks much better, might still not be a really good one, but for now, I like it.
redesign verify frame: do not cover plain text

the verify action is now shown in a small frame at the bottom of the signed
pgp message. It covers parts of the signature, but since this is binary,
there is no real problem. However it is still possible to read the plaintext
part of the message without closing the frame.
make verify signature button smaller

signatures might be quite short, 96px seems enough to me.