[plasmalnf] Document configuration

- Improve documentation, explain necessity of theme: and image: keys
 - Scale screenshot up with font size (numbers picked arbitrarily)

src/modules/plasmalnf/ThemeWidget.cpp

@@ -1,6 +1,6 @@
 /* === This file is part of Calamares - <https://github.com/calamares> ===
  *
- *   Copyright 2017, Adriaan de Groot <groot@kde.org>
+ *   Copyright 2017-2018, Adriaan de Groot <groot@kde.org>
  *
  *   Calamares is free software: you can redistribute it and/or modify
  *   it under the terms of the GNU General Public License as published by
@@ -20,6 +20,7 @@
 
 #include "ThemeInfo.h"
 
+#include "utils/CalamaresUtilsGui.h"
 #include "utils/Logger.h"
 
 #include <QHBoxLayout>
@@ -28,16 +29,18 @@
 
 ThemeWidget::ThemeWidget(const ThemeInfo& info, QWidget* parent)
     : QWidget( parent )
+    , m_id( info.id )
     , m_check( new QRadioButton( info.name.isEmpty() ? info.id : info.name, parent ) )
     , m_description( new QLabel( info.description, parent ) )
-    , m_id( info.id )
 {
     QHBoxLayout* layout = new QHBoxLayout( this );
     this->setLayout( layout );
 
     layout->addWidget( m_check, 1 );
 
-    constexpr QSize image_size{120, 80};
+    const QSize image_size{
+        qMax(12 * CalamaresUtils::defaultFontHeight(), 120), 
+        qMax(8 * CalamaresUtils::defaultFontHeight(), 80) };
 
     QPixmap image( info.imagePath );
     if ( info.imagePath.isEmpty() )

src/modules/plasmalnf/plasmalnf.conf

@@ -28,8 +28,20 @@ lnftool:	"/usr/bin/lookandfeeltool"
 #
 # Themes may be listed by id, (e.g. fluffy-bunny, below) or as a theme
 # and an image (e.g. breeze) which will be used to show a screenshot.
-# Themes with no image get a "missing screenshot" image; if the
+# Themes with no image set at all get a "missing screenshot" image; if the
 # image file is not found, they get a color swatch based on the image name.
+#
+# Valid forms of entries in the *themes* key:
+#  - A single string (unquoted), which is the theme id
+#  - A pair of *theme* and *image* keys, e.g.
+#    ```
+#    - theme: fluffy-bunny.desktop
+#      image: "fluffy-screenshot.png"
+#    ```
+#
+# The image screenshot is resized to 12x8 the current font size, with
+# a minimum of 120x80 pixels. This allows the screenshot to scale up
+# on HiDPI displays where the fonts are larger (in pixels).
 themes:
     - org.kde.fuzzy-pig.desktop
     - theme: org.kde.breeze.desktop