As noted in CR feedback, having a selector named `focusedGroup` which
returns a `Group` and a state property named `focusedGroup` is
`string|null` is confusing.

Also change a `g` variable to `group` for clarity.

Update the groups service to use the Redux store for storing and reading
groups state instead of storing it internally in private variables.

Add a Redux module that stores state related to groups, including the
currently focused group and the list of all known groups.

This is an initial step towards moving this state from the groups
service into the store, which will make reacting to changes in the set
of loaded groups, or the focused group, easier.

Extract `createStore` helper out of `store` service

Replace "inherits" package with ES 2015 class inheritance

Convert Adder to an ES2015 class

Remove the need to alias the `Adder` instance to `self` by using
arrow-functions.

This commit and the next few refactor `Adder` to a "proper" class for
easier readability.

Add `expand` param to all groups.list requests

Minor bug fix

Replace use of "inherits" package to create classes that derive from
`EventEmitter` with ES 2015 `class Foo extends EventEmitter` syntax, for
readability.

This method is part of the Redux store API so we don't need to test it
in detail, but this is additional verification that `createStore`
returns a working Redux store.

Move date, time & retry utility code to `util/` dir

Pdf subframe annotation

As per #648, move several modules containing utility functions which
have no dependencies on other code to `src/sidebar/util`.

The `thunk` middleware is used by the `annotations` module and likely
others in future. Rather than require it to be passed to `createStore`
in each test that uses it, I think it makes sense to always include it
as the standard way to handle async/non-pure actions in the client.

This functionality is now covered by tests for the `createStore` helper.

The `store` factory function contains some useful logic for composing a
store out of a set of _modules_ which handle different parts of the
application state, exposing the actions and selectors from each module
as methods on the store.

To facilitate re-use of this logic in tests and to make it easier to
test this logic directly, extract it out of the `store` function into a
separate `createStore` helper.

Doing this enables removal of `modules/index.js` which existed to
combine parts of the modules together.

Rename "annotationUI" => "store"

Rename the fake/stub implementations of the Redux store used in various
component and service tests.

Update the name of the identifier used for the Redux store in various
contexts, following the previous refactoring to move store-related code
into `store/`.

Move Redux modules and entry point to `store/` directory

Changed background to selected element in PDF.js to visualize the searched content

Remove reference to `group.url`

Remove `group.public` property from annotation component