Handle save-as on pane. Replace 'editor:save-as' w/ 'core:save-as'

Pane will only show the saveAs dialog if the item has a `saveAs` method.

src/app/keymaps/atom.cson

@@ -1,5 +1,6 @@
 'body':
   'meta-s': 'core:save'
+  'meta-S': 'core:save-as'
   'enter': 'core:confirm'
   'escape': 'core:cancel'
   'meta-w': 'core:close'

src/app/keymaps/editor.cson

@@ -2,7 +2,6 @@
   'meta-T': 'editor:undo-close-session'
 
 '.editor':
-  'meta-S': 'editor:save-as'
   'enter': 'editor:newline'
   'meta-enter': 'editor:newline-below'
   'meta-shift-enter': 'editor:newline-above'