Add some way to specify a directory to load dictionaries from at runtime

Currently Qt only looks for dictionaries in the application folder and the Qt library folder. This is insufficient for applications that might want to download and install dictionaries dynamically, or override shipping dictionaries. It also does not allow shipping of dictionaries with applications on platforms like linux where the applicationDir() would be /usr/bin

Either an environment variable or better a per-profile setting to add directories to the start of the search paths is needed.