AppKit: Add CPTextAttachment and image support to CPTextView

AppKit lacked a public text attachment class, preventing rich text documents
from embedding or inserting images.

* AppKit/CPTextView/CPTextAttachment.j:
  - Implement CPTextAttachment (modelled on NSTextAttachment) supporting
    initialization via data and file type, contents, file types, images,
    bounds, and keyed archiving.
  - Adopt a TextKit 2-style view rendering architecture that scales images
    to fit text container width and triggers layout updates upon asynchronous loading.

* AppKit/CPTextView/CPTextStorage.j:
  - Update attributedStringWithAttachment: to accept CPTextAttachment instances
    alongside direct views, routing rendering through existing attachment pathways.

* AppKit/CPTextView/CPTextView.j:
  - Implement the insertImage: action (invoking browser file pickers) and
    Command-V paste handling for images.
  - Configure inserted attachments with responsive view instances scaled to
    text width, omitting live views during pasteboard archiving.

* AppKit/CPTextView/CPTextContainer.j:
  - Adjust attachment bounds to match updated container widths during layout invalidation.

* AppKit/AppKit.j:
  - Export CPTextAttachment.

* Tests/AppKit/CPTextAttachmentTest.j:
  - Add test coverage for data URLs, archiving, width scaling, insertion,
    and serialized text output.

* Tests/Manual/TextEditor/Resources/MainMenu.xib, MainMenu.cib:
  - Add Edit > Insert Image… menu item.

* Tests/Manual/TextEditor/WORKLIST.md:
  - Mark image insertion testing as completed; log remaining image tasks as open.

Rich text documents now support embedded images inserted via file selection or clipboard paste.

AppKit/CPTextView/CPTextStorage.j

@@ -26,6 +26,7 @@
 
 @import "CPText.j"
 @import "CPFont.j"
+@import "CPTextAttachment.j"
 
 @class CPLayoutManager;
 
@@ -263,16 +264,23 @@ var CPTextStorageDelegate_textStorageWillProcessEditing_ = 1 << 1,
 }
 
 /*!
-    Returns an instance of CPTextStorage that contains the provided instance of CPView.
-    This can be used to insert arbitrary views into the text. These views are treated as individual characters during editing.
-    This works only with views that conform to the CPCoding protocol
+    Returns an instance of CPTextStorage that contains one attachment character for anAttachment.
+    The attachment is treated as an individual character during editing.
+
+    anAttachment is either a CPTextAttachment, as in Cocoa, or a view. A CPTextAttachment is
+    stored as the character's CPAttachmentAttributeName attribute, and its view is laid out as
+    the character. A view is inserted into the text directly; this works only with views that
+    conform to the CPCoding protocol.
 */
 
-+ (id)attributedStringWithAttachment:(CPView)someView
++ (id)attributedStringWithAttachment:(id)anAttachment
 {
     var result = [[self alloc] initWithString:_CPAttachmentCharacterAsString];
 
-    [result setAttributes:@{_CPAttachmentView:someView} range:CPMakeRange(0, 1)];
+    if ([anAttachment isKindOfClass:[CPTextAttachment class]])
+        [result setAttributes:@{CPAttachmentAttributeName:anAttachment, _CPAttachmentView:[anAttachment _attachmentView]} range:CPMakeRange(0, 1)];
+    else
+        [result setAttributes:@{_CPAttachmentView:anAttachment} range:CPMakeRange(0, 1)];
 
     return result;
 }