Persist window location and size

[fotijr]

This change allows the window's location, size, and state to persist. Windows settings are save on close and restored when opening again.

Notes:

• I don't have much experience with WPF, but I tried keeping my edits as minimal as possible. I used a few different approaches described in this SO post.
• This is my first github pull request, so let me know if anything doesn't look right.

[fotijr]

If you want, I could also add more info to the repo's README file. I could include the info you already have in the VS marketplace, as well as a GIF of the extension in action. But that's your call, just let me know if you want me to add it.

[parnic]

Great, thanks!

If you'd like to update the readme, that's fine with me.