Plugin: add extensible key/value pair list

This makes the annotation much more extensible, so that various sorts of
plugins can register various bits of information in a freeform way.
Remove ImageJ tool-specific plugin attributes

These include alwaysActive and activeInAppFrame. ImageJ can define them
using the new key/value pair extension mechanism instead.
Plugin: tweak javadoc
Plugin: remove superfluous cancelable attribute

This attribute was intended for use with ImageJ commands, but there is
already a dedicated interface called Cancelable for signaling whether
the command can be canceled. So the attribute is unnecessary.
Merge mirror-website project with scijava-common
Update description of SciJava Common library
Add license-maven-plugin configuration
Fix copyrights and licenses

This was done using the commands:

    mvn license:update-project-license
    mvn license:update-file-header
Remove ImageJ-isms

This is mostly in javadoc. But it also renames the ij.log.level property
to scijava.log.level instead.