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/dialog/preferences/tabs/preferencesgeneraltab.cpp

@@ -103,7 +103,7 @@ PreferencesGeneralTab::PreferencesGeneralTab()
 
     default_still_length_ = new FloatSlider();
     default_still_length_->SetMinimum(0.1);
-    default_still_length_->SetFormat(tr("%1 second(s)"));
+    default_still_length_->SetFormat(tr("%1 seconds"));
     default_still_length_->SetValue(Config::Current()["DefaultStillLength"].value<rational>().toDouble());
     timeline_layout->addWidget(default_still_length_);
 
@@ -136,7 +136,7 @@ PreferencesGeneralTab::PreferencesGeneralTab()
     autorecovery_interval_ = new IntegerSlider();
     autorecovery_interval_->SetMinimum(1);
     autorecovery_interval_->SetMaximum(60);
-    autorecovery_interval_->SetFormat(tr("%1 minute(s)"));
+    autorecovery_interval_->SetFormat(QT_TRANSLATE_N_NOOP("olive::SliderBase", "%n minute(s)"), true);
     autorecovery_interval_->SetValue(Config::Current()[QStringLiteral("AutorecoveryInterval")].toLongLong());
     autorecovery_layout->addWidget(autorecovery_interval_, row, 1);