[UX] Keyboard: add d diacritics (#4898)

Eth (ð) is an important letter for people who like Old English (me), and for IPA.

Ď/ď is Czech/Slovak.

Đ/đ is sometimes an alternative rendering of ð, maybe because you could type it on a traditional typewriter. It's also Slavic and something Vietnamese.

Delta is an often-used Greek letter.

frontend/ui/data/keyboardlayouts/en_keyboard.lua

@@ -8,6 +8,8 @@ local _B_ = en_popup._B_
 local _b_ = en_popup._b_
 local _E_ = en_popup._E_
 local _e_ = en_popup._e_
+local _D_ = en_popup._D_
+local _d_ = en_popup._d_
 local _S_ = en_popup._S_
 local _s_ = en_popup._s_
 
@@ -34,7 +36,7 @@ return {
         {  --  1           2       3       4       5       6       7       8       9       10      11      12
             { _A_,        _a_,    "…",    _at,    "Ф",    "ф",    "*",    "0",    "Ê",    "ê",    "Ş",    "ş", },
             { _S_,        _s_,    "$",    "4",    "Ы",    "ы",    "+",    "4",    "Ë",    "ë",    "İ",    "ı", },
-            { "D",        "d",    "%",    "5",    "В",    "в",    "-",    "5",    "Î",    "î",    "Ğ",    "ğ", },
+            { _D_,        _d_,    "%",    "5",    "В",    "в",    "-",    "5",    "Î",    "î",    "Ğ",    "ğ", },
             { "F",        "f",    "^",    "6",    "А",    "а",    "=",    "6",    "Ï",    "ï",    "Ć",    "ć", },
             { "G",        "g",    ":",    ";",    "П",    "п",    "Ү",    "ү",    "Ô",    "ô",    "Č",    "č", },
             { "H",        "h",    "\"",   "'",    "Р",    "р",    "Ұ",    "ұ",    "Œ",    "œ",    "Đ",    "đ", },

frontend/ui/data/keyboardlayouts/keypopup/en_popup.lua

@@ -65,6 +65,20 @@ return {
         northeast = "β",
         northwest = "♭",
     },
+    _D_ = {
+        "D",
+        north = "Đ",
+        northeast = "Ð",
+        northwest = "Ď",
+        east = "Δ",
+    },
+    _d_ = {
+        "d",
+        north = "đ",
+        northeast = "ð",
+        northwest = "ď",
+        east = "δ",
+    },
     _E_ = {
         "E",
         north = "Ë",