proxy: add dedicated Proxy dialog and preferences fields

- New ProxyDialog (Tools > Proxy Settings..., plus 'Proxy Settings...'
  in the project panel and timeline Proxy submenus) unifying global
  proxy settings, per-footage custom presets, generation and deletion
  in one place instead of three scattered entry points; this also fixes
  the Tools menu action opening the wrong preferences tab
- Preferences Disk tab gains an 'include audio in proxies' checkbox and
  an ffmpeg executable path field (blank = auto-detect)
- Add ProxyDialog smoke tests

app/dialog/CMakeLists.txt

@@ -30,6 +30,7 @@ if (OpenTimelineIO_FOUND)
 endif ()
 add_subdirectory(preferences)
 add_subdirectory(progress)
+add_subdirectory(proxy)
 add_subdirectory(projectproperties)
 add_subdirectory(rendercancel)
 add_subdirectory(sequence)