xquartz: Fix a possible crash when editing the Application menu due to mutaing immutable arrays

Crashing on exception: -[__NSCFArray replaceObjectAtIndex:withObject:]: mutating method sent to immutable object

Application Specific Backtrace 0:
0   CoreFoundation                      0x00007ff80d2c5e9b __exceptionPreprocess + 242
1   libobjc.A.dylib                     0x00007ff80d027e48 objc_exception_throw + 48
2   CoreFoundation                      0x00007ff80d38167b _CFThrowFormattedException + 194
3   CoreFoundation                      0x00007ff80d382a25 -[__NSCFArray removeObjectAtIndex:].cold.1 + 0
4   CoreFoundation                      0x00007ff80d2e6c0b -[__NSCFArray replaceObjectAtIndex:withObject:] + 119
5   X11.bin                             0x00000001003180f9 -[X11Controller tableView:setObjectValue:forTableColumn:row:] + 169

Fixes: https://github.com/XQuartz/XQuartz/issues/267


Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>