Rework variable tab to use tab helpers

lib/variables/variable-tab.hpp

@@ -0,0 +1,20 @@
+#pragma once
+#include "resource-table.hpp"
+
+namespace advss {
+
+class VariableTable final : public ResourceTable {
+	Q_OBJECT
+
+public:
+	static VariableTable *Create();
+
+private slots:
+	void Add();
+	void Remove();
+
+private:
+	VariableTable(QTabWidget *parent = nullptr);
+};
+
+} // namespace advss