Server API and Controller bugs

[d.healey]

There are still a few annoying bugs with the server stuff, I thought I'd put them all in a single thread. If you know of any more please add them below.

• downloadFile() triggers 2 downloads for a single file instead of 1.
• Server Panel pause and abort controls don't work properly, see here
• Sometimes downloads that fail will turned to pause status and can't be removed, in such cases HISE has to be restarted to clear it.
• Headers aren't passed with downloadFile requests, so downloads can't be "authorized"
• Downloads that have a status of waiting don't seem to be affected by the .stop, .resume or .abort commands.