Korektor Spellchecker

Allow using Korektor service on editable controls.

Korektor Spellchecker extension allows using Korektor service (spellchecker and occassional grammarchecker developed at Institute of Formal and Applied Linguistics, Charles University in Prague) on any editable input fields of web pages. The extension allows either directly correcting the content, or showing a dialog with suggested corrections. Note that the dialog with suggestions is injected directly into the original page, so there can be various problems on untested sites. However, the extension seem to work fine on many sites. Although the sources of Korektor service and this extension are available under BSD-3-Clause license, please respect the CC BY-SA-NC licence of the spellchecking models. Korekor service is described at https://lindat.mff.cuni.cz/services/korektor/ , the Korektor itself is available from http://ufal.mff.cuni.cz/korektor .