Make some identifiers more concise

See https://go.dev/doc/effective_go#package-names, specifically:

> The importer of a package will use the name to refer to its contents,
> so exported names in the package can use that fact to avoid
> repetition.

For example, `engines.GoogleTranslateEngine` needlessly repeats
"engine," so just get rid of that duplication by renaming it to
`engines.GoogleTranslate`.

Renaming `engines.TranslationEngine` to `engines.Engine` may be
debatable, so if somebody disagrees, feel free to leave a comment
stating your disagreement and with an explanation of why you disagree.

git-svn-id: file:///srv/svn/repo/mai/trunk@10 e410bdd4-646f-c54f-a7ce-fffcc4f439ae

engines/engine.go

@@ -5,7 +5,7 @@ type TranslationResult struct {
 	TranslatedText string
 }
 
-type TranslationEngine interface {
+type Engine interface {
 	InternalName() string
 	DisplayName() string
 	SourceLanguages() ([]Language, error)