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/de.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: 114.495, height: 52     }
     special:          { width: 40.41,   height: 52     }
@@ -38,7 +37,7 @@ views:
     symbols:
         - "Tab-small Ctrl Alt Shift ↑ ↓ ← →"
         - "~ ` · √ π τ ÷ × ¶ @"
-        - "-- © ® £ € ¥ ^ ° { } --"
+        - "\\ © ® £ € ¥ ^ ° { } _"
         - "show_numbers % | < > = [ ] BackSpace"
         - "show_letters show_eschars preferences space . Return"
     eschars:
@@ -267,7 +266,4 @@ buttons:
     Ü:
         outline: "subtle-highlight"
     ß:
-        outline: "subtle-highlight"
-    --:
-        outline: "placeholder"
-        text: ""
+        outline: "subtle-highlight"