Client-side validation for file upload sizes in gallery/asset multi-files upload, gallery/asset editor, image picker

The single file upload function using in gallery editor, asset editor were implemented in CMS7-8065. For image picker file upload, it was updated with validation service in CMS7-8632.

The multi-files upload in gallery/asset (trunk) is using jquery file-uploads plugin that uses only server-side validation.

Currently, they only use server-side validation service. Therefore users may need to upload a large file (e.g 5MB), then find out that its size is invalid (default limitation is 4MB). This way is not efficient for users with limited network bandwidth.

It'd better to implement another client-side validation, so users can be warned on selecting large files before upload. It can be done using File API and javascript.