Auto merge of #11029 - izgzhen:filemanager_thread, r=Manishearth

Add FileManagerThread

This is intended to support the File API implementation. Basically an event loop with three kinds of messages:

+ Select a file
+ Read a file with ID
+ Delete the ID from manager-owned map

The design decision in this PR is not the final (or best I think) version, welcome reviews :)

TODOs:

- [x] Add multiple file selection

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components/servo/Cargo.lock

@@ -1407,6 +1407,7 @@ dependencies = [
  "serde_macros 0.7.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "url 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "util 0.0.1",
+ "uuid 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "websocket 0.17.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
@@ -2383,6 +2384,7 @@ version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "rand 0.3.14 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde 0.7.4 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]