Add support for opening links in incognito mode on Linux & BSD (#4745)

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/util/XDGDirectory.hpp

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <QStringList>
+
+namespace chatterino {
+
+#if defined(Q_OS_UNIX) and !defined(Q_OS_DARWIN)
+
+enum class XDGDirectoryType {
+    Config,
+    Data,
+};
+
+/// getXDGDirectories returns a list of directories given a directory type
+///
+/// This will attempt to read the relevant environment variable (e.g. XDG_CONFIG_HOME and XDG_CONFIG_DIRS) and merge them, with sane defaults
+QStringList getXDGDirectories(XDGDirectoryType directory);
+
+#endif
+
+}  // namespace chatterino