Don't load the icon at creation time

This fixes one deadlock I've encountered twice. Main thread is holding the `iconLoadingSet` lock :
```
    1033 Thread_5679373   DispatchQueue_6: com.apple.root.default-priority  (concurrent)
    + 1033 ???  (in Quicksilver)  load address 0x106f8e000 + 0x1ce4  [0x106f8fce4]
    +   1033 NSApplicationMain  (in AppKit) + 940  [0x7fff879bb7a3]
    +     1033 -[NSApplication run]  (in AppKit) + 646  [0x7fff879d0a19]
    +       1033 ???  (in Quicksilver)  load address 0x106f8e000 + 0x318a  [0x106f9118a]
    +         1033 -[NSApplication sendEvent:]  (in AppKit) + 3395  [0x7fff87b80b32]
    +           1033 -[NSWindow sendEvent:]  (in AppKit) + 1763  [0x7fff87bdf51b]
    +             1033 -[NSWindow _processKeyboardUIKey:]  (in AppKit) + 353  [0x7fff87c80fe9]
    +               1033 -[QSSearchObjectView cancelOperation:]  (in QSInterface) + 315  [0x107159586]
    +                 1033 -[QSInterfaceController hideMainWindowFromCancel:]  (in QSInterface) + 92  [0x107141673]
    +                   1033 -[QSInterfaceController hideMainWindowWithEffect:]  (in QSInterface) + 355  [0x10714155d]
    +                     1033 -[NSNotificationCenter postNotificationName:object:userInfo:]  (in Foundation) + 68  [0x7fff8b6b67ba]
    +                       1033 _CFXNotificationPost  (in CoreFoundation) + 2893  [0x7fff943728dd]
    +                         1033 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__  (in CoreFoundation) + 12  [0x7fff9447ee0c]
    +                           1033 +[QSObject purgeImagesAndChildrenOlderThan:]  (in QSCore) + 159  [0x10709abbb]
    +                             1033 -[NSSet objectsWithOptions:passingTest:]  (in CoreFoundation) + 167  [0x7fff94407c47]
    +                               1033 __NSSetEnumerate  (in CoreFoundation) + 393  [0x7fff94407e59]
    +                                 1033 dispatch_apply_f  (in libdispatch.dylib) + 603  [0x7fff8b9d775b]
    +                                   1033 _dispatch_apply_invoke  (in libdispatch.dylib) + 87  [0x7fff8b9d782a]
    +                                     1033 _dispatch_client_callout2  (in libdispatch.dylib) + 8  [0x7fff8b9d78d9]
    +                                       1033 ____NSSetEnumerate_block_invoke  (in CoreFoundation) + 186  [0x7fff944b907a]
    +                                         1033 __40-[NSSet objectsWithOptions:passingTest:]_block_invoke  (in CoreFoundation) + 33  [0x7fff94408021]
    +                                           1017 _OSSpinLockLockSlow  (in libsystem_platform.dylib) + 63  [0x7fff92905df6]
    +                                           ! 1017 syscall_thread_switch  (in libsystem_kernel.dylib) + 10  [0x7fff94356b16]
    +                                           16 _OSSpinLockLockSlow  (in libsystem_platform.dylib) + 81,77,...  [0x7fff92905e08,0x7fff92905e04,...]
```
Background thread finds a QSCommand that contains a string as its indirect, triggers an icon load that get stuck against that same lock :
```
    1033 Thread_8698584   DispatchQueue_6: com.apple.root.default-priority  (concurrent)
    + 1033 start_wqthread  (in libsystem_pthread.dylib) + 13  [0x7fff8ec35fb9]
    +   1033 _pthread_wqthread  (in libsystem_pthread.dylib) + 314  [0x7fff8ec32ef8]
    +     1033 _dispatch_worker_thread2  (in libdispatch.dylib) + 40  [0x7fff8b9d3177]
    +       1033 _dispatch_root_queue_drain  (in libdispatch.dylib) + 326  [0x7fff8b9d2082]
    +         1033 _dispatch_client_callout  (in libdispatch.dylib) + 8  [0x7fff8b9d028d]
    +           1033 _dispatch_apply_invoke  (in libdispatch.dylib) + 87  [0x7fff8b9d782a]
    +             1033 _dispatch_client_callout2  (in libdispatch.dylib) + 8  [0x7fff8b9d78d9]
    +               1033 ____NSSetEnumerate_block_invoke  (in CoreFoundation) + 186  [0x7fff944b907a]
    +                 1033 __40-[NSSet objectsWithOptions:passingTest:]_block_invoke  (in CoreFoundation) + 53  [0x7fff94408035]
    +                   1033 -[__NSSetM addObject:]  (in CoreFoundation) + 408  [0x7fff943cf898]
    +                     1033 -[QSCommand isEqual:]  (in QSCore) + 262  [0x10708db25]
    +                       1033 -[QSCommand iObject]  (in QSCore) + 478  [0x10708d90d]
    +                         1033 +[QSObject(StringHandling) objectWithString:]  (in QSCore) + 68  [0x1070a9cf2]
    +                           1033 -[QSObject(StringHandling) initWithString:]  (in QSCore) + 178  [0x1070a9dc2]
    +                             1033 -[QSObject(Icon) loadIcon]  (in QSCore) + 102  [0x10709f0d1]
    +                               1033 -[QSObject(Accessors) setIconLoaded:]  (in QSCore) + 63  [0x10709e83c]
    +                                 1033 _pthread_mutex_lock  (in libsystem_pthread.dylib) + 372  [0x7fff8ec34779]
    +                                   1033 __psynch_mutexwait  (in libsystem_kernel.dylib) + 10  [0x7fff9435a746]
```