feat: basic plugin system

The wayland plugin now uses it.

src/wayland/init.cpp

@@ -0,0 +1,19 @@
+#include <qguiapplication.h>
+#include <qqml.h>
+
+#include "../core/plugin.hpp"
+#include "waylandlayershell.hpp"
+
+namespace {
+
+class WaylandPlugin: public QuickshellPlugin {
+	bool applies() override { return QGuiApplication::platformName() == "wayland"; }
+
+	void registerTypes() override {
+		qmlRegisterType<WaylandPanelInterface>("QuickShell", 1, 0, "PanelWindow");
+	}
+};
+
+QS_REGISTER_PLUGIN(WaylandPlugin);
+
+} // namespace