[libcalamares] Add convenience accessor for translations

- Which translations are available is a global property
   of Calamares itself, not of the plugins, so getting
   the model of available translations should live there.
   Move the relevant code (which is simple) from the
   Welcome module.

src/libcalamares/locale/LabelModel.cpp

@@ -18,6 +18,8 @@
 
 #include "LabelModel.h"
 
+#include "CalamaresVersion.h"  // For the list of translations
+
 namespace CalamaresUtils::Locale
 {
 
@@ -104,4 +106,10 @@ LabelModel::find( const QLocale& locale ) const
     } );
 }
 
+LabelModel* const availableTranslations()
+{
+    static LabelModel model( QString( CALAMARES_TRANSLATION_LANGUAGES ).split( ';') );
+    return &model;
+}
+
 }  // namespace

src/libcalamares/locale/LabelModel.h

@@ -30,7 +30,7 @@ namespace CalamaresUtils {}
 namespace CalamaresUtils::Locale
 {
 
-DLLEXPORT class LabelModel : public QAbstractListModel
+class DLLEXPORT LabelModel : public QAbstractListModel
 {
 public:
     enum
@@ -65,5 +65,17 @@ private:
     QVector< Label > m_locales;
 } ;
 
+/** @brief Returns a model with all available translations.
+ *
+ * The translations are set when Calamares is compiled; the list
+ * is provided by CMake via the CALAMARES_TRANSLATION_LANGUAGES
+ * #define.
+ *
+ * This model is a singleton and can be shared.
+ *
+ * NOTE: While the model is not typed const, it should be. Do not modify.
+ */
+DLLEXPORT LabelModel* const availableTranslations();
+
 }  // namespace
 #endif

src/modules/welcome/WelcomePage.cpp

@@ -131,7 +131,7 @@ WelcomePage::initLanguages()
     ui->languageWidget->clear();
     ui->languageWidget->setInsertPolicy( QComboBox::InsertAtBottom );
 
-    m_languages = new CalamaresUtils::Locale::LabelModel( QString( CALAMARES_TRANSLATION_LANGUAGES ).split( ';') );
+    m_languages = CalamaresUtils::Locale::availableTranslations();
     ui->languageWidget->setModel( m_languages );
     ui->languageWidget->setItemDelegate( new LocaleTwoColumnDelegate( ui->languageWidget ) );