Windows: Add "Is Selected" keyword

[ynerant]

Hi!

I am using the Windows library to automate a WPF application, and I wasn't able to check that a checkbox or a radiobutton was selected.

I tried to add a new keyword "Is Selected" that uses GetSelectionItemPattern to get the selection state. I copied what was done for the "Get Value" keyword.

I hope this will help! Feel free to edit my PR if there is something wrong.

[CLAassistant]

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