fix(HotKey): store only unique keys in key down array
fix(tests): change source file name
fix(HotKey): reset keys when app window lost focus

This could happen when user use Alt + Tab for example so the component stores key  in keys list (onKeyDown) but it doesn't remove it (onKeyUp event is not called)
refactor(HotKey): rename to HotKeys

BREAKING CHANGE: rename HotKey to HotKeys because it's more clear what it does
docs(exmaple): rename source file on example page
feat(HotKeysElement): move events handling to separate service

HotKeys is service to manage hotkeys business logic, not DOM events.

BREAKING CHANGE: HotKeys service no longer has any arguments in its factory.
test(ParseKey): test for failure if unknown or invalid expression is given