Added ability to take screenshot of a single window

[Mindar]

I added the ability to take screenshots of a window using the X-Server window ID (as talked about in #121 ). Obviously this only works on linux (so far).

I also added the function screenshot.listWindows(), which works similar to screenshot.listDisplays(). This function can be used to get a list of all windows' IDs. Windows returned by screenshot.listWindows() are ordered by X's "stacking order" (whatever that is supposed to mean... (I didn't research it further)).

Example:

const availableWindows = await screenshot.listWindows()
// availableWindows = ['0x4600001', '0x5600008', '0x360067a', '0x4800007', '0x4e00017', '0x3600003', ...]

const randomWindowId = availableWindows[4] // 4 was randomly chosen by fair dice roll

screenshot({windowId: randomWindowId}).then((img) => {
    // img: Buffer filled with jpg of the window
})

I'll try to add the same functionality for windows in the near future. Unfortunately I can't add it for Mac, because I don't own one.

Lastly, I added the actual MIT license text, so the project actually complies with its own license:

...
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
...

Feel free to change the attribution line. For now I set it to "Copyright (c) 2020 Ben Evans and screenshot-desktop contributors".

[Mindar]

It works on windows too now. I also found a pretty easy way to add window screenshots for MacOS, but I didn't find an easy way to get a list of Window IDs.

@bencevans Is there any chance of this getting merged without Mac support?

[bencevans]

That's great @Mindar. Would you mind adding some tests (spawning notepad.exe on windows? spawning firefox on Linux maybe?) and adding a bit of documentation to the README with an example and mentioning that support is windows/Linux with an OSX pull-request welcome?

[augustnmonteiro]

does this work if the window is minimized? like when we share something in a google meet,zoom,.. even if the window is minimized everyone else keeps seeing it.

[kriakiku]

Hello! Is this MR blocked by something?

[bencevans]

Documentation and Tests