CaptionUiElements takes too long and doesn't use parallelization correctly.

[sfetzel]

Fix for unit test TestCaptionUiElements_ShouldNotExceedTimeLimit in CaptionViewModelTest needed.

Expected behavior:
The method CaptionViewModel.CaptionUiElements() should under no circumstances take longer than 100ms (specified in property CaptionViewModel.CaptionTimeLimit).
(In addition: once the user pressed any key, the captioned elements must not change anymore.)

Actual behavior:
The method takes too long and does not execute all the tasks, which could be executed within the time limit.

[mfe-]

CaptionViewModel.CaptionUiElements[UpdateElements]