proxy: per-footage presets, audio in proxies, configurable ffmpeg path

- Footage can store custom proxy parameters (width/height/crf/preset/
  extension/audio) that override the global settings; they are
  serialized with the project and used by every generation entry point
  via Footage::GetEffectiveProxyParams()
- Proxies now include the source audio streams (AAC) unless disabled;
  the proxy filename records the audio flag and offline audio rendering
  decodes from the proxy when present
- ProxyTask resolves ffmpeg from the new FFmpegPath config key first,
  then PATH, then common install locations (e.g. Homebrew on macOS),
  instead of relying on PATH only; the error message points at the
  preferences when no executable is found
- ProxyTask::BuildArguments() is extracted for testability
- Add ProxyIncludeAudio and FFmpegPath config defaults
- Add regression tests for the filename audio marker, config-backed
  params, ffmpeg resolution, argument building, and custom-param
  persistence

app/widget/projectexplorer/projectexplorer.cpp

@@ -611,11 +611,7 @@ void ProjectExplorer::GenerateProxiesForSelectedFootage()
 			continue;
 		}
 
-		ProxyManager::ProxyParams params;
-		params.width = OLIVE_CONFIG("ProxyWidth").value<int>();
-		params.height = OLIVE_CONFIG("ProxyHeight").value<int>();
-		params.crf = OLIVE_CONFIG("ProxyCRF").value<int>();
-		params.preset = OLIVE_CONFIG("ProxyPreset").toString();
+		ProxyManager::ProxyParams params = item->GetEffectiveProxyParams();
 		const ProxyManager::Proxy proxy =
 			ProxyManager::instance()->GetOrStartProxy(
 				item->project()->cache_path(), item->filename(),