Add a very basic x11/standalone platform plugin

The plugin does not much. It's the most basic plugin we can have to
be loaded from kwin_x11.

Unlike the wayland platform plugins it gets installed to:
org.kde.kwin.platforms

plugins/platforms/x11/CMakeLists.txt

@@ -1,3 +1,4 @@
+add_subdirectory(standalone)
 if(X11_XCB_FOUND)
     add_subdirectory(windowed)
 endif()

plugins/platforms/x11/standalone/CMakeLists.txt

@@ -0,0 +1,13 @@
+set(X11PLATFORM_SOURCES
+    x11_platform.cpp
+)
+
+add_library(KWinX11Platform MODULE ${X11PLATFORM_SOURCES})
+target_link_libraries(KWinX11Platform kwin Qt5::X11Extras)
+
+install(
+    TARGETS
+        KWinX11Platform
+    DESTINATION
+        ${PLUGIN_INSTALL_DIR}/org.kde.kwin.platforms/
+)

plugins/platforms/x11/standalone/x11.json

@@ -0,0 +1,8 @@
+{
+    "KPlugin": {
+        "Description": "Platform plugin for standalone x11 in kwin_x11.",
+        "Id": "KWinX11Platform",
+        "Name": "x11-standalone"
+    },
+    "input": true
+}

plugins/platforms/x11/standalone/x11_platform.cpp

@@ -0,0 +1,44 @@
+/********************************************************************
+ KWin - the KDE window manager
+ This file is part of the KDE project.
+
+Copyright (C) 2016 Martin Gräßlin <mgraesslin@kde.org>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*********************************************************************/
+#include "x11_platform.h"
+
+#include <QX11Info>
+
+namespace KWin
+{
+
+X11StandalonePlatform::X11StandalonePlatform(QObject *parent)
+    : Platform(parent)
+{
+}
+
+X11StandalonePlatform::~X11StandalonePlatform() = default;
+
+void X11StandalonePlatform::init()
+{
+    if (!QX11Info::isPlatformX11()) {
+        emit initFailed();
+        return;
+    }
+    setReady(true);
+    emit screensQueried();
+}
+
+}

plugins/platforms/x11/standalone/x11_platform.h

@@ -0,0 +1,45 @@
+/********************************************************************
+ KWin - the KDE window manager
+ This file is part of the KDE project.
+
+Copyright (C) 2016 Martin Gräßlin <mgraesslin@kde.org>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*********************************************************************/
+#ifndef KWIN_X11STANDALONE_PLATFORM_H
+#define KWIN_X11STANDALONE_PLATFORM_H
+#include "platform.h"
+
+#include <kwin_export.h>
+
+#include <QObject>
+
+namespace KWin
+{
+
+class KWIN_EXPORT X11StandalonePlatform : public Platform
+{
+    Q_OBJECT
+    Q_INTERFACES(KWin::Platform)
+    Q_PLUGIN_METADATA(IID "org.kde.kwin.Platform" FILE "x11.json")
+public:
+    X11StandalonePlatform(QObject *parent = nullptr);
+    virtual ~X11StandalonePlatform();
+    void init() override;
+
+};
+
+}
+
+#endif