Call Bool before evaluating {enabled/visible}_when condition for labels

[aaronayres35]

closes #1768

WIP label because the still needs a test. support was introduced in #1544, but that needs a test as well. Not sure if I will have the time to write a test on this, but wanted to open a PR with the simple fix now so it isn't forgetten. (previous comment about difficulty I had writing tests for Labels {enabled/visible}_when #1544 (comment))

Note that for "normal" enabled/visible when we have cond_value come up in an if statement:

traitsui/traitsui/ui.py

Line 793 in 87dceb2

if cond_value and (at_init or not editor_state):

In the code here we really do want to always pass a bool to setVisible or setEnabled.

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[aaronayres35]

@corranwebster this PR was labelled as a WIP because it does not contain a test but it is a trivial change that fixes the associated issue. I had trouble writing a test for Label {enabled/visible}_when previously (#1544 (comment)) and am not sure I'll have the time to figure that out now. Wanted to get your thoughts on if you think it is safe / worth it to throw this fix into the next release or if we should just hold off.

[corranwebster]

LGTM