Add helper file common.hpp

The file currently contains function `qS(...)` which is an alias for
`QString::fromStdString(...)`

src/common.hpp

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <QString>
+
+#include <string>
+
+namespace chatterino {
+
+inline QString qS(const std::string &string)
+{
+    return QString::fromStdString(string);
+}
+
+}  // namespace chatterino