BUG: Update CTK to fix Qt designer crash at startup

One of the DICOM widgets crashed the Qt designer because
`ctkDICOMScheduler::~ctkDICOMScheduler()` called `ctkJobScheduler::waitForDone`,
and that method called `processEvents()`. After returning from `processEvents()`
the application crashed because the "this" pointer has become invalid (probably
the `ctkDICOMScheduler` object got deleted).

Solved the issue by removing `processEvents()` from `ctkJobScheduler::waitForDone`,
and instead calling `processEvents()` before calling `ctkJobScheduler::waitForDone`.

List of CTK changes:

```
$ git shortlog 4aba4e203..51c9f4a5a --no-merges
Andras Lasso (1):
      BUG: Fix Qt designer crash at startup
```

Co-authored-by: Andras Lasso <lasso@queensu.ca>

SuperBuild/External_CTK.cmake

@@ -71,7 +71,7 @@ if(NOT DEFINED CTK_DIR AND NOT Slicer_USE_SYSTEM_${proj})
 
   ExternalProject_SetIfNotDefined(
     Slicer_${proj}_GIT_TAG
-    "4aba4e20341c7111f5637337952a089464dc15db"
+    "51c9f4a5a55f38ddce9d8b75a72c02fca64bf17a"
     QUIET
     )