Added the `http` package as a direct dependency;

Implemented:
1. `LanguageFetchService` interface;
2. `LangFetchService` standard implementation;
3. `CachingLangFetchService` decorator.

Added the Dropdown to the example

To remove the dependency on the `language_tool`

to remove the dependency on the `language_tool` package;

Enabled changing the spellcheck language on the fly in the `ColoredTextEditingController`.
Implementing #25.