Attachments Refactoring

[mhdostal]

This refactors the Existing popup attachment views so they can be used by both Popups and Feature Forms. Functionality includes:

• View all attachments in a feature
• Display default icon appropriate for attachment types (unloaded attachments)
• Ability to load attachments by tapping on them
• Display custom attachment thumbnails (loaded attachments; certain types only - images, movies, PDFs)
• Display editing controls for adding/deleting/renaming attachments (editing functionality is still a work-in-progress)
• Ability to view attachments full-screen using native file viewer

Code changes include:

• Add FeatureAttachment protocol, inherited by both PopupAttachment and FormAttachment
• Add AttachmentsFeatureElement protocol, inherited by both AttachmentsPopupElement and AttachmentFormElement
• Refactor existing popup element/attachment views to use the new FeatureAttachment and AttachmentsFeatureElement protocols, allowing both popups and Forms to use them.
• Renamed existing file AttachmentsPopupElementView to AttachmentsFeatureElementView
• Moved common UI/model files to new "Common" folder under "ArcGISToolkit"
• Add context menu for "Rename" and "Delete" of attachments (Form editing-only)
• Add "Add" button for adding new attachments (Form editing-only)
• Added a shouldEnableEditControls view modifier to AttachmentsFeatureElementView in order to enable/display editing controls.
• In most cases, I reordered the #imports.
• Removed if !targetEnvironment(macCatalyst) checks for QuickLook stuff as it now appears that is supported on macOS.

[mhdostal]

I've addressed all your feedback. I added most of the suggestions; where I didn't I added comments.

[mhdostal]

Excellent suggestions, thank you. Ready for re-review.