fix(ngShowHide): stop ngShow and ngHide applying animations incorrectly

[grantyb]

Fix ngShow and ngHide so they don't animate in/out their classes every time the watch expression is evaluated. These directives now only change when the watch expression's boolean value actually changes.

Closes #4401

[mary-poppins]

Thanks for the PR!

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Thanks again for your help!

[grantyb]

Thanks Mary :-) I just signed a CLA - My full name is Grant Berridge.
I also updated the commit message. I hope it now fulfils the requirements.

[matsko]

Replaced by: #4612

My reasoning is explained here: #4401 (comment)