Menu improvements

Currently, the application menu looks like the following:

─┬─ File ─┬─ Open video
 │        ├─ Open project
 │        ├─ Save project
 │        ├─ Export…
 │        └─ Quit
 ├─ Play ─┬─ Play/Pause
 │        ├─ Stop
 │        ├─ Go to previous frame
 │        ├─ Go to previous fifth frame
 │        ├─ Go to previous tenth frame
 │        ├─ Go to next frame
 │        ├─ Go to next fifth frame
 │        └─ Go to next tenth frame
 ├─ Edit ─┬─ Add Timeline line
 │        └─ Start/Finish Annotation
 ├─ View ─── Toggle Fullscreen
 └─ Help ─── About

Improvements should be made:

• The entry File/Export… should be changed into File/Export CSV File, or something more explicit.
• The Edit submenu should be populated with the new capabilities, eventually breaking it down into two subsubmenus:
  • Timeline: Add new, Delete, Change label, Edit events
  • Annotation: Start/finish new, Select/Deselect, Change event, Add comment
• In the Help menu, add entries for going to the PyPI and the GitLab webpages
• The menu entries should be enough to inform the user about the available keybindings.