Implemented CSSStyleDeclaration.setPropertyPriority

Implemented CSSStyleDeclaration.setPropertyPriority, resolves #4433
auto merge of #4471 : ProgramFOX/servo/issue-4433, r=jdm

Implemented CSSStyleDeclaration.setPropertyPriority, resolves #4433.
Add GetPropertyPriority to CSSStyleDeclaration

Tweak getPropertyPriority to match recommendations

Adding Tests for Style Priority

Use else if

Adding Content Test for GetPropertyPriority

Revert "Adding Tests for Style Priority"

This reverts commit 8666a37f833b06c3e43f27acd8a9678e94425e55.
auto merge of #4463 : iterion/servo/get-property-priority, r=jdm

Implementation of #4432 adding `getPropertyPriority` to CSSStyleDeclaration. This is my first attempt at a Servo PR so I'm sure I've done something wrong. Let me know, and I'll fix it up.

As stated in #4432 tests for this are in #4085. If there are additional tests I can write now I would love to do that, I'm just not sure where or what those would be.
This fixes #4259. Dispatching the mousemove event for the top most node when the mouse moves while on top of it
Allow selection of all text in a text control using the ctrl-a/cmd-a shortcut.

Fixes #4411.
Replace XHR events for generic ones in ScriptTask

fixup! Replace XHR events for generic ones in ScriptTask

fixup! Replace XHR events for generic ones in ScriptTask
auto merge of #4453 : mattnenterprise/servo/dispatch-mousemove, r=jdm
auto merge of #4457 : jamougha/servo/master, r=jdm

Fixes #4411.

I didn't find any tests for the text control functionality so I didn't write any for this, let me know if I should.
auto merge of #4474 : thiagopnts/servo/generic-msgs, r=jdm

This refs #3735. As discussed in the issue, I did it cloning when I couldn't dereference an attribute. The trait method should be on `&self` for object-safety reasons.