More Cleaning

- MODTIME is string, MODTIME_RAW is integer. Use the latter one for `const time_t source_modtime`
- Clearify GMENUITEM_LINE and GMENUITEM2_LINE
- Comment out a number of unused function
- Fix startui.c for Mac (oops)
- The `dlist_free_external(&hklist)` might have been a bug, please review!