Add Google and Unsplash terms links to experimental/desktop_photo_search

[domesticmouse]

Add links to the following in an about dialog or similar:

• https://policies.google.com/terms
• https://unsplash.com/terms

[SatYu26]

@domesticmouse Hello sir. I have made few changes according to the requirement in this issue.
As of now the dialog looks like this:

I have added sentences according to the information available to me but if you let me know the specific details of the statements i will make those changes quickly.

Also i have added the access point for this dialog in About menu inside the menubar, As you can see in this picture.

I will can make more changes also if required by you.

shall i create PR?