FileSystemWatcher: add a removeAllFolders() method

[yfede]

More a feature request really.

Use case:

I have a list in my GUI showing the files in a folder.
Said list gets updated when the FileSystemWatcher says something has changed.
The user can press a "browse" a button to change which folder has its content listed.
When the folder to display changes, I must tell the FileSystemWatcher to stop watching the old folder and watch the new one.
This implies that at that moment I must have a track of what the old folder was to call removeFolder(), while what I really would like to do is tell the watcher: "forget whatever folder you were watching before", and then call addFolder(newFolder) to watch the new one

[FigBug]

Added FileSystemWatcher::removeAllFolders()