fixes #159

Might also help with #47 & #105

DMS hides directories that don't contain any "files of interest". The way this is currently implemented is that when a directory is browsed, readContainer is called, which, via cdsObjectToUpnpavObject, objectChildCount calls readContainer on all child directories. Further, when encountering a file, cdsObjectToUpnpavObject calls ffmpegProbe. This means that ffmpegProbe is called on all files in the whole directory subtree on which Browse/BrowseDirectChildren was called. This is also the case when BrowseDirectChildren is called on the root path.

While DMS won't do a full library scan on startup, it will do a full scan on the first BrowseDirectChildren call it receives, likely on the root path.

It seems to me that this was implemented, at least in part, to hide folders that don't contain files of interest. It needs to recursively check child folders to see if there's any files of interest, though in doing so it scans all files in all subdirectories, which technically isn't needed for this use case.

This PR addresses the issue having a different code path for determining what subdirectories to show/hide. Currently, when cdsObjectToUpnpavObject is called on a directory, it calls objectChildCount to determine whether the current folder is of interest. This in turn calls readDir, which calls cdsObjectToUpnpavObject, which calls ffmpegProbe. The proposed change makes objectChildCount no longer call readContainer, but instead iterate over the contained files and directories directly, calling a new func isOfInterest on each to determine whether that object should count towards the child count of the containing folder. isOfInterest calls func objectHasChildren which was updated to no longer call objectChildCount, which would result in the recursion, but instead has the same "is of interest" checks that cdsObjectToUpnpavObject uses. This means it will also walk the directory tree, and will call ffmpegProbe only on files in direct subdirectories for which BrowseDirectChildren was called, instead of all files in the tree.

Old:

graph TD;
    BrowseDirectChildren-->readContainer;
    readContainer-->cdsObjectToUpnpavObject;
    cdsObjectToUpnpavObject-->|isDirectory| objectChildCount;
    objectChildCount-->readContainer;
    cdsObjectToUpnpavObject-->|isFile| ffmpegProbe;

New:

graph TD;
    BrowseDirectChildren-->readContainer;
    readContainer-->cdsObjectToUpnpavObject;
    cdsObjectToUpnpavObject-->|isFile| ffmpegProbe;
    cdsObjectToUpnpavObject-->|isDirectory| objectChildCount;
    objectChildCount-->isOfInterest;
    isOfInterest-->|isDirectory| objectHasChildren;
    objectHasChildren-->isOfInterest