Fix unfound video definition because of sensitive comparaison.

• Remove used deprecated functions on video_definition and add missing linphone_core_set_preview_video_definition_by_name
• Add an API to get recommended video definitions to make a difference between what definitions Linphone support, and what definitions that are suitable for a widest set of hardware for all video codec implementations in order to excludes some very high definition formats that are unlikely to work unless specific hardware or codecs are used.
• Directory API : allow to set data, config and download paths from factory. Keep fallback on old computation if paths are not set by the application.
• Update log file only if enabled to avoid unwanted file creation. A workaround before this fix is to set prefix before path.

(backport from release/5.1) liblinphone!1682 (merged)