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/widget/nodeparamview/nodeparamviewarraywidget.cpp

@@ -53,7 +53,7 @@ void NodeParamViewArrayWidget::UpdateCounter(const QString& input, int old_size,
 {
   Q_UNUSED(old_size)
   if (input == input_) {
-    count_lbl_->setText(tr("%1 element(s)").arg(new_size));
+    count_lbl_->setText(tr("%n element(s)", nullptr, new_size));
   }
 }