Use std::vector<string> instead of QStringList in main()

In an effort to convert core to C++ structures.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/subsurface-string.cpp

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "subsurface-string.h"
+
+std::string join(const std::vector<std::string> &l, const std::string &separator, bool skip_empty)
+{
+	std::string res;
+	bool first = true;
+	for (const std::string &s: l) {
+		if (skip_empty && l.empty())
+			continue;
+		if (!first)
+			res += separator;
+		res += s;
+		first = false;
+	}
+	return res;
+}