Fix thumbnails not shown in Windows and excessive use of virtual memory /...

Fix thumbnails not shown in Windows and excessive use of virtual memory / handles which may cause crashes and other unexpected behavior by removing file monitoring and only monitoring directories for changes. These problems might still occur with very large numbers of folders but could not be confirmed. However, medium to large collections should no longer cause a problem.

BUG: 290962
CCBUG: 297793
CCBUG: 308310
CCBUG: 310252
CCBUG: 310865
CCBUG: 312422
CCBUG: 312999
CCBUG: 291917
CCBUG: 295445
CCBUG: 297686
FIXED-IN: 3.0.0