The #queryCommand: method is one of the least satisfactory elements of the
original MVP design, because it results in a large and often complex and
difficult to maintain switch statement (albeit a switch on value, and not
type). It also doesn't lend itself to extensibility, and consequently
various ad-hoc approaches have been used to achieve that, e.g. registering
additional handlers in the dev tool presenter hierarchy, and the use of
ClosedCommandDescriptions (commands that have their own query block, and
which are invoked first, effectively shortcutting the normal
chain-of-command route).

This commit adds a prototypical replacement mechanism using method
annotations to associate a query function with a command. The annotation
is placed in the command method and specifies the selector of the query
function for that command. A single query method can be associated with
multiple commands.

In order to test out the mechanism in practice, the #queryCommand of
ClassSelector has been replaced. This is a relatively complex example, and
so should cover many of the variations. A couple of extension cases have
also been implemented with the new mechanism. It is, however, expected that
other cases will be encountered as more #queryCommand: methods are replaced, and
so the design may require refinement.

All uses of the existing extension mechanism that allowed for provision of
extra old-style command query methods, e.g. the queryRefactoringCommand:
methods added to PackageBrowserShell, etc, have also been replaced.