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/slider/sliderbase.cpp

@@ -39,6 +39,7 @@ SliderBase::SliderBase(Mode mode, QWidget *parent) :
   dragged_diff_(0),
   require_valid_input_(true),
   tristate_(false),
+  format_plural_(false),
   drag_ladder_(nullptr),
   ladder_element_count_(0),
   dragged_(false)
@@ -101,9 +102,10 @@ bool SliderBase::IsDragging() const
   return drag_ladder_;
 }
 
-void SliderBase::SetFormat(const QString &s)
+void SliderBase::SetFormat(const QString &s, const bool plural)
 {
   custom_format_ = s;
+  format_plural_ = plural;
   ForceLabelUpdate();
 }
 
@@ -113,6 +115,11 @@ void SliderBase::ClearFormat()
   ForceLabelUpdate();
 }
 
+bool SliderBase::IsFormatPlural() const
+{
+  return format_plural_;
+}
+
 void SliderBase::ForceLabelUpdate()
 {
   UpdateLabel(Value());
@@ -209,6 +216,8 @@ void SliderBase::UpdateLabel(const QVariant &v)
 {
   if (tristate_) {
     label_->setText("---");
+  } else if (format_plural_) {
+    label_->setText(tr(GetFormat().toUtf8().constData(), nullptr, v.toInt()));
   } else {
     label_->setText(GetFormat().arg(ValueToString(v)));
   }

app/widget/slider/sliderbase.h

@@ -60,9 +60,11 @@ public:
 
   bool IsDragging() const;
 
-  void SetFormat(const QString& s);
+  void SetFormat(const QString& s, const bool plural=false);
   void ClearFormat();
 
+  bool IsFormatPlural() const;
+
   void SetLadderElementCount(int b)
   {
     ladder_element_count_ = b;
@@ -127,6 +129,8 @@ private:
 
   QString custom_format_;
 
+  bool format_plural_;
+
   SliderLadder* drag_ladder_;
 
   int ladder_element_count_;