Interacting with opened windows

[michallepicki]

Closes #360

TODO:

• Expose through Selenium, Chrome and Browser modules
• Check no window_handle vs "current" window_handle (spec says it's "current"), possibly change get_window_size and set_window_size to use it as well
• Revisit handling errors, reading the value from the response and return types (:ok is just the happy path) that's fine for now I think
• Test in Selenium: Safari 11, Edge 17, Firefox, Chrome, Opera, Appium (older version compatible with this protocol)
• Test in Chromedriver with a few Chrome and Chromium versions
• Add documentation

For the unaware: I'm following the Selenium WebDriver Wire Protocol. The W3C WebDriver Protocol has too many changes to try to support both of them at the same time. We will need to have a separate WebdriverClient module for it for people that want to test in Safari 12 or newer, Edge 18 or newer, new Appium versions etc.

[michallepicki]

I think getting and setting window size or position using a handle is not supported in many WebDrivers. Since the user can always change focus to that window first before executing these commands, I'm going to simplify this PR and remove the versions that take a handle. I'll keep the change from "no handle" to "current" to designate the target window since it seems to be working fine and it's more spec compliant with the old protocol.

[michallepicki]

@mhanberg @keathley I think this is good enough!

I didn't add an integration test for maximize_window because this command requires a window manager to be running.

A few existing commands got slightly changed to be more in-line with the Selenium WebDriver spec (w3c endpoints were being used), versions that take a window handle got dropped for simplicity (and because they returned values for current window for many browsers anyway), and return values got adjusted so that we always return {:ok, value} from the WebDriverClient module when the command succeeded.