Remove manual qmlcachegen invocation for internal Qt QML files

It's not safe to always assume we can always precompile everything.
Instead set QML_CACHE_DIR to ensure the files go to our desired location.

app/path.h

@@ -7,8 +7,8 @@ class Path
 {
 public:
     static QString getLogDir();
-
     static QString getBoxArtCacheDir();
+    static QString getQmlCacheDir();
 
     static QByteArray readDataFile(QString fileName);
     static void writeCacheFile(QString fileName, QByteArray data);
@@ -24,4 +24,5 @@ private:
     static QString s_CacheDir;
     static QString s_LogDir;
     static QString s_BoxArtCacheDir;
+    static QString s_QmlCacheDir;
 };