OCRTool: selection in UI should only list available languages

[ewerybody]

Split off issue from a2#250

Currently the combobox in the OCRTool ui shows en-US de-DE, FirstFromAvailableLanguages.

This might cause issues if one of these is in fact not installed! So the

• ui should reflect whats actually there! Plus:
• offer a button to goto the dialog/website to download language packs