AppKit: Let an editable text view take the Tab key

CPWindow handled every Tab key press itself and moved to the next key view, so a text view never received Tab and could not insert a tab character.

It now follows Cocoa: an editable text view takes plain Tab and inserts a tab, while Control-Tab, Shift-Tab and Tab in other views still move through the key view loop.

* CPResponder.j: Add _wantsTabKey, which returns NO by default.

* CPTextView/CPTextView.j: Want the Tab key while editable.

* CPWindow/CPWindow.j: Pass plain Tab to a first responder that wants it, and keep the browser from moving its own focus.

AppKit/CPTextView/CPTextView.j

@@ -1839,6 +1839,16 @@ Sets the selection to a range of characters in response to user action.
     [self insertText:@"\t"];
 }
 
+/*
+    @ignore
+    As in Cocoa, an editable text view takes the Tab key and inserts a tab character.
+    Control-Tab still moves to the next key view.
+*/
+- (BOOL)_wantsTabKey
+{
+    return [self isEditable];
+}
+
 - (void)insertTabIgnoringFieldEditor:(id)sender
 {
     [self insertTab:sender];