Mouse-friendly functions bar for meters panel

Before:
SpaceStyle EnterMove  DelDeleteEscDone  |
~~~~~      ~~~~~      ~~~      ~~~      |
UpUp     DnDown  LtLeft  RtRight EnterConfirmDelDeleteEscDone  |
~~       ~~      ~~      ~~      ~~~~~       ~~~      ~~~      |

After:
SpaceStyle EnterMove                                           DelDeleteF10Done
~~~~~      ~~~~~      ~~                                       ~~~      ~~~
SpaceStyle EnterLock  UpUp    DnDown  <-Left  ->Right          DelDeleteF10Done
~~~~~      ~~~~~      ~~      ~~      ~~      ~~      ~~       ~~~      ~~~

* Align 'Delete' and 'Done' to the right to match functions on other
  screens. (Accidental clicking is avoided as a side benefit.)
* You could change meter type while in moving mode. New bar now hints
  this.
* Two Enter key functions are put in the same place and so mouse clicks
  there act like functions toggle. (The wording change to 'Lock' is
  also to reflect this.)
* '<-' and '->' instead of 'Lt' and 'Rt' abbreviation as the latter is
  not widely seen and arrows shapes are obvious. :)
* 'Esc' key for 'Done' in this context may not be intuitive, comparing
  to 'F10'. While I wish there be a Cancel/Undo function for 'Esc', it
  wouldn't hurt if we write 'F10' for 'Done' on functions bar for now.

MetersPanel.h

@@ -29,6 +29,10 @@ struct MetersPanel_ {
 
 // Note: In code the meters are known to have bar/text/graph "Modes", but in UI
 // we call them "Styles".
+// We avoid UTF-8 arrows ← → here as they might display full-width on Chinese
+// terminals, breaking our aligning.
+// In <http://unicode.org/reports/tr11/>, arrows (U+2019..U+2199) are
+// considered "Ambiguous characters".
 
 void MetersPanel_setMoving(MetersPanel* this, bool moving);