more control on what shell is loaded

Summary:
Since shellManager was removed, we have a quite big regressions
of any other shell that isn't the desktop one not loading in any
way from SDDM, causing a big issue for plasma mobile or any shell
based on plasma mini shell, which we have several now not working
images based on.

this allows for a shell being configured in plasmarc

Test Plan: shell specified in plasmarc loaded, loads the desktop if not specified

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23031

shell/main.cpp

@@ -88,6 +88,11 @@ int main(int argc, char *argv[])
 
     app.setQuitOnLastWindowClosed(false);
 
+
+    KSharedConfig::Ptr startupConf = KSharedConfig::openConfig(QStringLiteral("plasmarc"));
+    KConfigGroup startupConfGroup(startupConf, "Shell");
+    const QString defaultShell = startupConfGroup.readEntry("ShellPackage", "org.kde.plasma.desktop");
+
     {
     QCommandLineParser cliOptions;
 
@@ -101,7 +106,7 @@ int main(int argc, char *argv[])
 
     QCommandLineOption shellPluginOption(QStringList() << QStringLiteral("p") << QStringLiteral("shell-plugin"),
                                          i18n("Force loading the given shell plugin"),
-                                         QStringLiteral("plugin"), QStringLiteral("org.kde.plasma.desktop"));
+                                         QStringLiteral("plugin"), defaultShell);
 
     QCommandLineOption standaloneOption(QStringList() << QStringLiteral("a") << QStringLiteral("standalone"),
                                         i18n("Load plasmashell as a standalone application, needs the shell-plugin option to be specified"));