copy / paste of tags doesn't copy all tags

[griff]

When copy / pasting tags only some tags are copied as reported by #61 and the following gets written to the log:

Tag 'duration' is read only
2013-04-15 17:55:20.826 MetaZ[13651:60b] (
    0   CoreFoundation                      0x00007fff8cd36f56 __exceptionPreprocess + 198
    1   libobjc.A.dylib                     0x00007fff8bad2d5e objc_exception_throw + 43
    2   CoreFoundation                      0x00007fff8cd36d8a +[NSException raise:format:arguments:] + 106
    3   CoreFoundation                      0x00007fff8cd36d14 +[NSException raise:format:] + 116
    4   MetaZKit                            0x00000001000b0bdc -[MZTimeCodeTag convertObjectForStorage:] + 65
    5   MetaZKit                            0x00000001000aba1c -[MetaEdits setterValue:forKey:] + 105
    6   MetaZKit                            0x00000001000ad429 -[MZDynamicObject setValue:forUndefinedKey:] + 156
    7   Foundation                          0x00007fff92bc1cd4 _NSSetUsingKeyValueSetter + 108
    8   Foundation                          0x00007fff92bc17d9 -[NSObject(NSKeyValueCoding) setValue:forKey:] + 400
    9   MetaZ                               0x000000010000788b -[FilesTableView paste:] + 1306
    10  CoreFoundation                      0x00007fff8cd2670d -[NSObject performSelector:withObject:] + 61
    11  AppKit                              0x00007fff8fd138ca -[NSApplication sendAction:to:from:] + 139
    12  AppKit                              0x00007fff8fe007ff -[NSMenuItem _corePerformAction] + 399
    13  AppKit                              0x00007fff8fe00536 -[NSCarbonMenuImpl performActionWithHighlightingForItemAtIndex:] + 125
    14  AppKit                              0x00007fff8fd7e0d4 -[NSMenu performKeyEquivalent:] + 281
    15  AppKit                              0x00007fff8fd7cd25 -[NSApplication _handleKeyEquivalent:] + 526
    16  AppKit                              0x00007fff8fc75e86 -[NSApplication sendEvent:] + 4282
    17  AppKit                              0x00007fff8fc0ca0e -[NSApplication run] + 555
    18  AppKit                              0x00007fff8fe88eac NSApplicationMain + 867
    19  MetaZ                               0x00000001000015b0 start + 52
)