Add prefix support for external proxies required for GoPro footage

This also adds the GoPro LRV proxy format definition.

data/externalproxies.rc

@@ -8,4 +8,5 @@
 
 [proxy]
 Sony PXW=../Sub;;S03.MP4;../Clip;;.MXF
+GoPro LRV=./;GL;.LRV;./;GX;.MP4
 

src/bin/projectclip.cpp

@@ -560,6 +560,10 @@ bool ProjectClip::setProducer(std::shared_ptr<Mlt::Producer> producer)
                     QDir dir = info.absoluteDir();
                     dir.cd(externalParams.at(3));
                     QString fileName = info.fileName();
+                    if (fileName.startsWith(externalParams.at(1))) {
+                        fileName.remove(0, externalParams.at(1).size());
+                        fileName.prepend(externalParams.at(4));
+                    }
                     if (!externalParams.at(2).isEmpty()) {
                         fileName.chop(externalParams.at(2).size());
                     }

[Farid Abdelnour]

Hey Alex, thanks for your commit! :)