[libcalamares] Implement reload-stylesheet

- From the debug-window, clicking *reload stylesheet* does just
   that, and applies the new stylesheet to the Calamares window.
 - Remove stylesheet caching from the Branding class; we only
   need the sheet once (on Calamares startup) or when updating
   the stylesheet, which is seldom-enough that we don't need
   to keep an extra copy around.
 - To use, start Calamares, open the debug window, open stylesheet.qss
   in an editor. Make changes, save, then click *reload stylesheet*.

SEE #1149

src/libcalamaresui/Branding.cpp

@@ -239,16 +239,6 @@ Branding::Branding( const QString& brandingFilePath,
         m_translationsPathPrefix.append( QString( "%1calamares-%2" )
                                             .arg( QDir::separator() )
                                             .arg( m_componentName ) );
-
-        QFileInfo importQSSPath( componentDir.filePath( "stylesheet.qss" ) );
-        if ( importQSSPath.exists() && importQSSPath.isReadable() )
-        {
-            QFile stylesheetFile( importQSSPath.filePath() );
-            stylesheetFile.open( QFile::ReadOnly );
-            m_stylesheet = stylesheetFile.readAll();
-        }
-        else
-            cWarning() << "the branding component" << componentDir.absolutePath() << "does not ship stylesheet.qss.";
     }
     else
     {
@@ -310,6 +300,21 @@ Branding::image( Branding::ImageEntry imageEntry, const QSize& size ) const
     return pixmap;
 }
 
+QString
+Branding::stylesheet() const
+{
+    QFileInfo fi( m_descriptorPath );
+    QFileInfo importQSSPath( fi.absoluteDir().filePath( "stylesheet.qss" ) );
+    if ( importQSSPath.exists() && importQSSPath.isReadable() )
+    {
+        QFile stylesheetFile( importQSSPath.filePath() );
+        stylesheetFile.open( QFile::ReadOnly );
+        return stylesheetFile.readAll();
+    }
+    else
+        cWarning() << "The branding component" << fi.absoluteDir().absolutePath() << "does not ship stylesheet.qss.";
+    return QString();
+}
 
 void
 Branding::setGlobals( GlobalStorage* globalStorage ) const

src/libcalamaresui/Branding.h

@@ -123,8 +123,13 @@ public:
     QString styleString( Branding::StyleEntry styleEntry ) const;
     QString imagePath( Branding::ImageEntry imageEntry ) const;
     QPixmap image( Branding::ImageEntry imageEntry, const QSize& size ) const;
-    /** @brief Stylesheet to apply for this branding. May be empty. */
+    /** @brief Stylesheet to apply for this branding. May be empty.
-    QString stylesheet() const { return m_stylesheet; }
+     *
+     * The file is loaded every time this function is called, so
+     * it may be quite expensive -- although normally it will be
+     * called only once, on startup. (Or from the debug window)
+     */
+    QString stylesheet() const;
 
     bool welcomeStyleCalamares() const { return m_welcomeStyleCalamares; }
     bool welcomeExpandingLogo() const { return m_welcomeExpandingLogo; }
@@ -151,14 +156,13 @@ private:
 
     [[noreturn]] void bail( const QString& message );
 
-    QString m_descriptorPath;
+    QString m_descriptorPath;  // Path to descriptor (e.g. "/etc/calamares/default/branding.desc")
-    QString m_componentName;
+    QString m_componentName;  // Matches last part of full path to containing directory
     QMap< QString, QString > m_strings;
     QMap< QString, QString > m_images;
     QMap< QString, QString > m_style;
     QString m_slideshowPath;
     QString m_translationsPathPrefix;
-    QString m_stylesheet;  // Text from file
 
     /** @brief Initialize the simple settings below */
     void initSimpleSettings( const YAML::Node& doc );