Move the development line to 0.4.x-dev: branch aliases, the packages'
requirements on each other (^0.4@dev), and the root path repository
versions. The packages first published with this release then require
the 0.4 framework they were written for, instead of accepting 0.3.0.

Record 0.4.0 in the changelog, with the missing-view exception as its
one breaking change, and turn the unreleased upgrade notes into the
steps from 0.3 to 0.4, adding the new optional packages.

Every package's composer.json links to the website. The skeleton does
not, because new projects start from its composer.json.