* Store all proxy clips in the same folder: $HOME/.cache/kdenlive/proxy to...

* Store all proxy clips in the same folder: $HOME/.cache/kdenlive/proxy to allow reusing proxy in different projects
* Proxy are now saved with a reference to size and extension, so you can keep several proxy versions depending on your proxy profile
* Fix proxy clips not rebuild when switching project's proxy profile
TODO: delete proxies when the proxy profile is edited and allow deletion of proxies
CCBUG: 364655