[plasmalnf] Add auto-detection of Plasma theme.

- Although it's not necessarily accurate for an extensively-modified
   Plasma configuration, we can read the Look-and-Feel from the
   configuration files. Allows auto-detection.

src/modules/plasmalnf/CMakeLists.txt

@@ -4,8 +4,13 @@ find_package(ECM ${ECM_VERSION} REQUIRED NO_MODULE)
 # needs a runtime support component (which we don't test for).
 set( lnf_ver 5.41 )
 
+find_package( KF5Config ${lnf_ver} )
 find_package( KF5Plasma ${lnf_ver} )
 find_package( KF5Package ${lnf_ver} )
+set_package_properties(
+    KF5Config PROPERTIES
+    PURPOSE "For finding default Plasma Look-and-Feel"
+)
 set_package_properties(
     KF5Plasma PROPERTIES
     PURPOSE "For Plasma Look-and-Feel selection"
@@ -16,11 +21,19 @@ set_package_properties(
 )
 
 if ( KF5Plasma_FOUND AND KF5Package_FOUND )
-    find_package( KF5 ${lnf_ver} REQUIRED CoreAddons Plasma Package )
+    if ( KF5Config_FOUND )
+        set( option_kf5 Config )
+        set( option_defs WITH_KCONFIG )
+        # set( option_libs KF5::Config )  # Not needed anyway
+    endif()
+
+    find_package( KF5 ${lnf_ver} REQUIRED CoreAddons Plasma Package ${option_kf5} )
 
     calamares_add_plugin( plasmalnf
         TYPE viewmodule
         EXPORT_MACRO PLUGINDLLEXPORT_PRO
+        COMPILE_DEFINITIONS
+            ${option_defs}
         SOURCES
             PlasmaLnfViewStep.cpp
             PlasmaLnfPage.cpp
@@ -32,6 +45,7 @@ if ( KF5Plasma_FOUND AND KF5Package_FOUND )
             page_plasmalnf.ui
         LINK_PRIVATE_LIBRARIES
             calamaresui
+            ${option_libs}
             KF5::Package
             KF5::Plasma
         SHARED_LIB

src/modules/plasmalnf/PlasmaLnfViewStep.cpp

@@ -26,8 +26,25 @@
 #include <QProcess>
 #include <QVariantMap>
 
+#ifdef WITH_KCONFIG
+#include <KConfigGroup>
+#include <KSharedConfig>
+#endif
+
 CALAMARES_PLUGIN_FACTORY_DEFINITION( PlasmaLnfViewStepFactory, registerPlugin<PlasmaLnfViewStep>(); )
 
+static QString
+currentPlasmaTheme()
+{
+#ifdef WITH_KCONFIG
+    KConfigGroup cg( KSharedConfig::openConfig( QStringLiteral( "kdeglobals" ) ), "KDE" );
+    return cg.readEntry( "LookAndFeelPackage", QString() );
+#else
+    cWarning() << "No KConfig support, cannot determine Plasma theme.";
+    return QString();
+#endif
+}
+
 PlasmaLnfViewStep::PlasmaLnfViewStep( QObject* parent )
     : Calamares::ViewStep( parent )
     , m_widget( new PlasmaLnfPage )
@@ -140,6 +157,8 @@ PlasmaLnfViewStep::setConfigurationMap( const QVariantMap& configurationMap )
     QString preselect;
     if ( configurationMap.contains( "preselect" ) && configurationMap.value( "preselect" ).type() == QVariant::String )
         preselect = configurationMap.value( "preselect" ).toString();
+    if ( preselect == QStringLiteral( "*" ) )
+        preselect = currentPlasmaTheme();
     if ( !preselect.isEmpty() )
         m_widget->setPreselect( preselect );
 

src/modules/plasmalnf/plasmalnf.conf

@@ -38,4 +38,8 @@ themes:
 # If this key is present, its value should be the id of the theme
 # which should be pre-selected. If absent, empty, or the pre-selected
 # theme is not found on the live system, no theme will be pre-selected.
-preselect: org.kde.breeze.desktop
+#
+# As a special setting, use "*", to try to find the currently-
+# selected theme by reading the Plasma configuration. This requires
+# KF5::Config at build- and run-time.
+preselect: "*"