Fix FileAPI's refcount implementation
Revise several intricate parts of FileAPI's internal refcounting-related implementation.
Goal: Get it done right once and for all.
r? @Manishearth
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1. More complete origin check in FileManagerThreadMsg
2. Add reference counting logic to file manage store and script API
3. Integrate the support of slicing
Changes include:
- Implement file selection and other DOM behaviours in htmlinputelement
- Integrate IpcSender<FileManagerThreadMsg> into ResourceThreads
- Improve filemanager_thread, including adding type_string field to SelectedFile
- Improve interfaces in FileList/File/Blob to accommodate the above changes
Changes include:
- Introduce an IpcSend trait to abstract over a collection of IpcSenders
- Implement ResourceThreads collection to abstract the resource-related
sub threads across the component
- Rename original ResourceThread and ControlMsg into an unifed CoreResource__
to accommodate above changes and avoid confusions
