layouts: Fill placeholders with duplicated buttons in terminal-layouts

to reduce changes when switching views.

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/703>

data/keyboards/terminal/us.yaml

@@ -8,7 +8,6 @@ outlines:
     halfwidth:     { width: 20.205,  height: 52     }
     large:         { width: 60.615,  height: 52     }
     narrow:        { width: 30.307,  height: 52     }
-    placeholder:   { width: 20.205,  height: 52     }
     small:         { width: 50.512,  height: 27.725 }
     spaceline:     { width: 148.17,  height: 52     }
     special:       { width: 60.615,  height: 52     }
@@ -37,7 +36,7 @@ views:
     symbols:
         - "Tab-small Ctrl Alt Shift ↑ ↓ ← →"
         - "~ ` · √ π τ ÷ × ¶ @"
-        - "-- © ® £ € ¥ ^ ° { } --"
+        - "\\ © ® £ € ¥ ^ ° { } _"
         - "show_numbers_from_symbols % | < > = [ ] BackSpace"
         - "show_letters preferences space . Return"
     actions:
@@ -233,6 +232,3 @@ buttons:
         outline: "halfwidth"
     _:
         outline: "halfwidth"
-    --:
-        outline: "placeholder"
-        text: ""