Freesound fixes for #5853, and a few other small improvements

Add a 'More' button to load the next page of results without clearing the already-found list.
Don't allow cancellation of searches, and don't update progress bar around searches, since we only get one page at a time now.
Show number of pages of results remaining to download in the tooltip of the 'More' button.
Use a new Mootcher object for each request, to avoid bad things happening when clicking in the Freesound search results list while a search or file download is already in progress.
Make the 'Stop' button insensitive except when it will actually stop the download of a sound file.
Only retrieve one page worth of data per search, rather than looping to get all pages.
Don't show an error in the log window if the user cancelled download.
Request 100 items per page, rather than the default 30.
Fix DOS line endings.