Replace old action API used by refactorings with newer handler API

The names of refactoring classes that provide an integration into the UI
of Eclipse have changed, in order to minimize overhead in the class
names.

The previous code contained some logic that should disable a menu item
when nothing is selected in the editor. But the existing logic was
wrong, it did never work. Because I did not find out how to implement it
correctly with Eclipse core-expressions I romved the logic completely.

Formerly, the context menu item, that showed the refactorings, contained
an item to the "refactoring qick menu", which has now been deleted. This
menu contains all available refactoring and it can be display by
entering a key combination. This item doesn't provide additional
information, thus the deletion. Users can always look up the key
combination in the preferences.

This also changes the ids of the refactring commands.

Fixes #1002049
Introduce a migration manager for key preferences/bindings

Whenever the command ids of key bindings, that are defined by users,
are changed all the preferences are lost because the new commands do no
longer consider their values. To avoid this problem the class
`MigrationPreferenceInitializer` is introduced which can copy old
preference values that are defined by users and set them as default
values for the new preferences.

This class also undefines the old command id and therefore the old key
binding after the migration is done.
Merge EditorHelpers into EditorUtils

The content of EditorHelpers is directly copy-pasted without any
functional changes.

Only some minor formatting changes and the addition of explicit
return types for public methods are applied.
Add property tester for text selections

The `nonEmpty` test on a text selection checks whether something is
selected in an editor. This is used in refactoring handlers in order
to disable them when they can't be applied.