[libcalamares] Support calling a slot on retranslation

- sometimes a slot is easier than a lambda. Introduce
   a macro CALAMARES_RETRANSLATE_SLOT that calls a given
   slot in an object on language change.
 - extend the retranslator with support for calling slots:
   - use Qt's signal/slot mechanism alongside the private
     list of functions to call
   - provide convenience for obtaining the retranslator of
     an object.

src/libcalamares/utils/Retranslator.cpp

@@ -118,8 +118,8 @@ translatorLocaleName()
     return s_translatorLocaleName;
 }
 
-void
-Retranslator::attachRetranslator( QObject* parent, std::function< void( void ) > retranslateFunc )
+Retranslator*
+Retranslator::retranslatorFor( QObject* parent )
 {
     Retranslator* r = nullptr;
     for ( QObject* child : parent->children() )
@@ -127,16 +127,17 @@ Retranslator::attachRetranslator( QObject* parent, std::function< void( void ) >
         r = qobject_cast< Retranslator* >( child );
         if ( r )
         {
-            break;
+            return r;
         }
     }
 
-    if ( !r )
-    {
-        r = new Retranslator( parent );
-    }
+    return new Retranslator( parent );
+}
 
-    r->m_retranslateFuncList.append( retranslateFunc );
+void
+Retranslator::attachRetranslator( QObject* parent, std::function< void( void ) > retranslateFunc )
+{
+    retranslatorFor( parent )->m_retranslateFuncList.append( retranslateFunc );
     retranslateFunc();
 }
 
@@ -159,6 +160,7 @@ Retranslator::eventFilter( QObject* obj, QEvent* e )
             {
                 func();
             }
+            emit languageChange();
         }
     }
     // pass the event on to the base

src/libcalamares/utils/Retranslator.h

@@ -46,10 +46,17 @@ class Retranslator : public QObject
 {
     Q_OBJECT
 public:
+    /// @brief Call @p retranslateFunc when the language changes
     static void attachRetranslator( QObject* parent, std::function< void( void ) > retranslateFunc );
+    /// @brief What retranslator belongs to @p parent (may create one)
+    static Retranslator* retranslatorFor( QObject* parent );
 
+    /// @brief Call @p retranslateFunc when the language changes
     void addRetranslateFunc( std::function< void( void ) > retranslateFunc );
 
+signals:
+    void languageChange();
+
 protected:
     bool eventFilter( QObject* obj, QEvent* e ) override;
 
@@ -65,5 +72,12 @@ private:
 #define CALAMARES_RETRANSLATE( body ) CalamaresUtils::Retranslator::attachRetranslator( this, [=] { body } );
 #define CALAMARES_RETRANSLATE_WIDGET( widget, body ) \
     CalamaresUtils::Retranslator::attachRetranslator( widget, [=] { body } );
+#define CALAMARES_RETRANSLATE_SLOT( slotfunc ) \
+    { \
+        this->connect( CalamaresUtils::Retranslator::retranslatorFor( this ), \
+                       &CalamaresUtils::Retranslator::languageChange, \
+                       this, \
+                       slotfunc ); \
+    }
 
 #endif