Added correct next/previous tab keys for Linux (as in Chromium). Previous default keys did not work for me at least.

src/files.h

@@ -97,8 +97,13 @@ const std::string configjson =
 "        \"run_command\": \"<alt>Return\",\n"
 "        \"kill_last_running\": \"<primary>Escape\",\n"
 "        \"force_kill_last_running\": \"<primary><shift>Escape\",\n"
+#ifdef __linux
+"        \"next_tab\": \"<primary>Page_Down\",\n"
+"        \"previous_tab\": \"<primary>Page_Up\",\n"
+#else
 "        \"next_tab\": \"<primary><alt>Right\",\n"
 "        \"previous_tab\": \"<primary><alt>Left\",\n"
+#endif
 "        \"close_tab\": \"<primary>w\"\n"
 "    },\n"
 "    \"project\": {\n"