qt: fix platform specific file dialog filters

Translations now only contain translation for the description and
extensions are generated in code taking account platform specific
differences.

src/qt/qt_util.cpp

@@ -0,0 +1,31 @@
+#include <QStringBuilder>
+#include <QStringList>
+#include "qt_util.hpp"
+
+namespace util
+{
+
+    QString DlgFilter(std::initializer_list<QString> extensions, bool last)
+    {
+        QStringList temp;
+
+        for (auto ext : extensions)
+        {
+#ifdef Q_OS_UNIX
+            if (ext == "*")
+            {
+                temp.append("*");
+                continue;
+            }
+            temp.append("*." % ext.toUpper());
+#endif
+            temp.append("*." % ext);
+        }
+
+#ifdef Q_OS_UNIX
+        temp.removeDuplicates();
+#endif
+        return " (" % temp.join(' ') % ")" % (!last ? ";;" : "");
+    }
+
+}