[plasmalnf] Improve theme-listing handling

- if key is missing or badly typed, enable all (explicitly)
 - document settings and code

src/modules/plasmalnf/PlasmaLnfPage.cpp

@@ -77,16 +77,20 @@ PlasmaLnfPage::setLnfPath( const QString& path )
 void
 PlasmaLnfPage::setEnabledThemes(const ThemeInfoList& themes)
 {
-    if ( themes.isEmpty() )
-        m_enabledThemes = plasma_themes();
-    else
-        m_enabledThemes = themes;
+    m_enabledThemes = themes;
 
     updateThemeNames();
     winnowThemes();
     fillUi();
 }
 
+void
+PlasmaLnfPage::setEnabledThemesAll()
+{
+    setEnabledThemes( plasma_themes() );
+}
+
+
 void PlasmaLnfPage::updateThemeNames()
 {
     auto plasmaThemes = plasma_themes();

src/modules/plasmalnf/PlasmaLnfPage.h

@@ -33,6 +33,12 @@ namespace Ui
 class PlasmaLnfPage;
 }
 
+/** @brief Page for selecting a Plasma Look-and-Feel theme.
+ *
+ * You must call setEnabledThemes -- either overload -- once
+ * to get the selection widgets. Note that calling that with
+ * an empty list will result in zero (0) selectable themes.
+ */
 class PlasmaLnfPage : public QWidget
 {
     Q_OBJECT
@@ -40,7 +46,10 @@ public:
     explicit PlasmaLnfPage( QWidget* parent = nullptr );
 
     void setLnfPath( const QString& path );
+    /** @brief enable only the listed themes. */
     void setEnabledThemes( const ThemeInfoList& themes );
+    /** @brief enable all installed plasma themes. */
+    void setEnabledThemesAll();
 
 signals:
     void plasmaThemeSelected( const QString& id );

src/modules/plasmalnf/PlasmaLnfViewStep.cpp

@@ -158,6 +158,8 @@ PlasmaLnfViewStep::setConfigurationMap( const QVariantMap& configurationMap )
             cDebug() << "WARNING: only one theme enabled in plasmalnf";
         m_widget->setEnabledThemes( allThemes );
     }
+    else
+        m_widget->setEnabledThemesAll();  // All of them
 }
 
 void

src/modules/plasmalnf/plasmalnf.conf

@@ -12,12 +12,14 @@ lnftool:	"/usr/bin/lookandfeeltool"
 # liveuser:	"live"
 
 # You can limit the list of Plasma look-and-feel themes by listing ids
-# here. If this key is not present, or the list is empty, all of the
-# installed themes are listed. If only one theme is listed, why are
-# you using this module at all? 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.
+# here. If this key is not present, all of the installed themes are listed.
+# If the key is present, only installed themes that are *also* included
+# in the list are shown (could be none!).
 #
+# 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
+# image file is not found, they get a color swatch based on the image name.
 themes:
     - org.kde.fuzzy-pig.desktop
     - theme: org.kde.breeze.desktop