Remove appbase as an external dependency.

lib/appbase/util/CombinePath.hpp

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <QDir>
+#include <QString>
+
+namespace chatterino {
+
+// https://stackoverflow.com/a/13014491
+inline QString combinePath(const QString &a, const QString &b)
+{
+    return QDir::cleanPath(a + QDir::separator() + b);
+}
+
+}  // namespace chatterino