Extract OS clipboard integration into embedding layer

[jdm]

Rather than implicitly supporting the OS clipboard everywhere by creating a ClipboardContext in the constellation, this is really something that should be supported separately by each Servo embedder.

We should:

• remove the GetClipboardContents/SetClipboardContents constellation messages
• add GetClipboardContents and SetClipboardContents messages to EmbedderMsg
• make the code that uses the clipboard provider in textinput.rs use Window's send_to_embedder method instead
• add new methods for getting/setting the clipboard contents to HostTrait
• call these new methods from the embedding API's handle_servo_events
• make the code that processes the new embedder clipboard messages call these new methods
• move the constellation's ClipboardContext creation to the Browser structure
• handle the new embedder clipboard messages in the glutin port's handle_servo_events by using the stored ClipboardContext

Code:

• components/script/clipboard_provider.rs
• components/script/textinput.rs
• components/constellation/constellation.rs
• components/embedder_traits/lib.rs
• components/script_traits/script_msg.rs
• ports/glutin/browser.rs
• ports/libsimpleservo/api/src.rs

There are no automated tests for this; it's enough to manually test that copying and pasting inside of a <input> inside the desktop Servo build still works before and after these changes.

[mmiecz]

Hi, this looks like good first task to get to know Servo! @jdm can you assign this one to me, please?

[mmiecz]

I've been looking into this task for some time, and I am not really sure how to call Windows send_to_embedder method from textinput.rs.
Are there any similar use-cases where I can look and see how I should approach this?

[jdm]

@mmiecz The implementation of send_to_embedder sends a ScriptMsg::ForwardToEmbedder message to the constellation. The current implementation of the clipboard provider uses a channel to the constellation, so you should be able to send the ForwardToEmbedder message directly in clipboard_provider.rs instead.

[jdm]

@mmiecz Are you still working on this?

[mmiecz]

@jdm Yes, I’ve done the sending to embedder, and removing the constellation part, and the clipboard is functional. I am not at home right now, but I will issue a PR soon