Fix plural handling for translations (#1424)

* .arg() doesn't handle plurals, replace %1 with %n and pass number to tr(), but only if there is an accompanying word for that we want singular/plural translations and only for whole numbers.

* Qt's translation methods only accept integers and we don't know what grammatical number form would be correct for floating-point numbers in different languages anyway.

* Add optional 2nd argument to SliderBase::SetFormat() to activate deferred plural handling:
  SetFormat(QT_TRANSLATE_N_NOOP("olive::SliderBase", "%n unit(s)"), true)

* Generate en_US.ts with lupdate's -pluralonly option. Only source strings that require plural handling need to be translated (as long as the source language is American English).

app/task/project/import/import.cpp

@@ -41,7 +41,7 @@ ProjectImportTask::ProjectImportTask(ProjectViewModel *model, Folder *folder, co
 
   file_count_ = Core::CountFilesInFileList(filenames_);
 
-  SetTitle(tr("Importing %1 file(s)").arg(file_count_));
+  SetTitle(tr("Importing %n file(s)", nullptr, file_count_));
 }
 
 const int &ProjectImportTask::GetFileCount() const