Refactor attachment file icons

[mronkko]

[mronkko]

This is somewhat of a bigger change, but relatively isolated.

The attachment file views are shown in four places:

1. The item list
2. The attachment carousel
3. The file receiving dialog
4. Conflict dialog

Icons are currently implemented as follows:

• Rendering the file type icons and PDF previews is done in ZPAttachmentIconImageFactory.
• The file name label and progress bars are managed in ZPAttachmentCarouselDelegate

These should be combined into one UIView subclass that can be just added to the view hierarchy and configured with an attachment item. The class should be configurable to listen to uploads, downloads, and deletions. If the view has user interaction enabled, tapping the view should start a download of the file or show the file in the file viewer if it is already downloaded.

This commit 162bf0f09b4f4d7c0f3afe2c4c2706355898b1a3 adds a skeleton for this new view.