scanner: make the scanner more thread safe

This fixes a crash due to concurrent changes of the FileTracks collection during scanning. Easy to trigger by holding down Delete.
Fix crash due to non-existing file being loaded into details pane

If a file is deleted just before being loaded into the details pane a crash will happen due to Windows being unable to read the file length. By wrapping this call in a try-catch the crash can be avoided.
properties: fix crash in properties window when two meta values are null (fix issue #2)

In the method which checks if two files have identical meta data, add a special case when both meta data values are null and consider these equal.