added clear button

[aldrindlsantos]

Hi @mariomka,

Please see the code change below. This will add a clear button on the datetime picker:

[mariomka]

Thank you.

I still miss some things:

• Add a test to test if the clear button works
• Boolean option to activate it (false by default)

On the other hand, some tests are failing.

[mixset]

@aldrindlsantos
Will you improve your pull request to make tests pass?
Otherwise @mariomka - Could you add such option?

[mixset]

@mariomka
Please take a look at above PR. When there will be next release?