[libcalamaresui] Support os-release substitutions in images

- Branding images might want to use os-release data as well.
 - Refactor a little to keep the number of #ifdefs the same;
   an intermediate expand() lambda handles expansion (or not,
   if it's not enabled).

FIXES #1153

src/branding/default/branding.desc

@@ -83,6 +83,8 @@ strings:
 #       any size and proportion, and will be scaled to fit inside
 #       the window. Use `welcomeExpandingLogo` to make it non-scaled.
 #       Recommended size is 320x150.
+#
+# These strings can also use substitutions from os-release (see above).
 images:
     productLogo:         "squid.png"
     productIcon:         "squid.png"

src/libcalamaresui/Branding.cpp

@@ -152,6 +152,7 @@ Branding::Branding( const QString& brandingFilePath,
             initSimpleSettings( doc );
 
 #ifdef WITH_KOSRelease
+            // Copy the os-release information into a QHash for use by KMacroExpander.
             KOSRelease relInfo;
 
             QHash< QString, QString > relMap{
@@ -173,19 +174,18 @@ Branding::Branding( const QString& brandingFilePath,
                 { QStringLiteral( "VARIANT_ID" ), relInfo.variantId() },
                 { QStringLiteral( "LOGO" ), relInfo.logo() }
             } };
-            loadStrings( m_strings, doc, "strings",
-                [&]( const QString& s ) -> QString { return KMacroExpander::expandMacros( s, relMap, QLatin1Char( '@' ) ); }
-                );
+            auto expand = [&]( const QString& s ) -> QString { return KMacroExpander::expandMacros( s, relMap, QLatin1Char( '@' ) ); };
 #else
-            // No support for substituting in os-release values.
-            loadStrings( m_strings, doc, "strings",
-                []( const QString& s ) -> QString { return s; }
-                );
+            auto expand = []( const QString& s ) -> QString { return s; };
 #endif
+
+
+            // Massage the strings, images and style sections.
+            loadStrings( m_strings, doc, "strings", expand );
             loadStrings( m_images, doc, "images",
                 [&]( const QString& s ) -> QString
                 {
-                    QFileInfo imageFi( componentDir.absoluteFilePath( s ) );
+                    QFileInfo imageFi( componentDir.absoluteFilePath( expand( s ) ) );
                     if ( !imageFi.exists() )
                         bail( QString( "Image file %1 does not exist." ).arg( imageFi.absoluteFilePath() ) );
                     return  imageFi.absoluteFilePath();