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/node/project/footage/footage.cpp

@@ -295,10 +295,9 @@ QString Footage::DescribeVideoStream(const VideoParams &params)
 
 QString Footage::DescribeAudioStream(const AudioParams &params)
 {
-  return tr("%1: Audio - %2 Channel(s), %3Hz")
-      .arg(QString::number(params.stream_index()),
-           QString::number(params.channel_count()),
-           QString::number(params.sample_rate()));
+  return tr("%1: Audio - %n Channel(s), %2Hz", nullptr, params.channel_count())
+    .arg(QString::number(params.stream_index()),
+         QString::number(params.sample_rate()));
 }
 
 void Footage::Hash(const QString& output, QCryptographicHash &hash, const rational &time) const