add sign-22.png to chrome manifest

this fixes the icon not being displayed in an inline encryption-frame of
the target web page.
fix logic which password dialog to open when signing

after a specific key is selected, the user is either in webmail mode
or external editor mode. Signing is nothing to do with decryption settings.
Now the test is correct and a popup is opened for webmail mode and the
password dialog is shown inline when in external editor mode.
add messageAdaptor for signDialog in firefox plugin

needed to sign messages in firefox
define openpgp and its util class in controller

needed for firefox to have openpgp object available, which is needed for
signing.
fix position of inline password dialog in editor

use the css from decryptPopup, so it is styled in the same way. Once css-files
are re-organized with less or sass, there can be a central file for styling
the password dialog.