[UX] Keyboard popups: stubs for the rest of the vowels (I, O, U) (#4907)

With the basic pattern of diacritics and special characters established by now, the basic diacritics + IPA don't need too much thought for an initial draft.

Cf. https://github.com/koreader/koreader/pull/4906#discussion_r274966796

frontend/ui/data/keyboardlayouts/en_keyboard.lua

@@ -10,10 +10,16 @@ local _E_ = en_popup._E_
 local _e_ = en_popup._e_
 local _D_ = en_popup._D_
 local _d_ = en_popup._d_
+local _I_ = en_popup._I_
+local _i_ = en_popup._i_
+local _O_ = en_popup._O_
+local _o_ = en_popup._o_
 local _S_ = en_popup._S_
 local _s_ = en_popup._s_
 local _T_ = en_popup._T_
 local _t_ = en_popup._t_
+local _U_ = en_popup._U_
+local _u_ = en_popup._u_
 
 return {
     shiftmode_keys = {["Shift"] = true},
@@ -29,9 +35,9 @@ return {
             { "R",        "r",    "#",    "3",    "К",    "к",    "~",    "3",    "ß",    "ß",    "4",    "¿", },
             { _T_,        _t_,    "+",    "=",    "Е",    "е",    "Ә",    "ә",    "À",    "à",    "5",    "¼", },
             { "Y",        "y",    "€",    "(",    "Н",    "н",    "І",    "і",    "Â",    "â",    "6",    "½", },
-            { "U",        "u",    "‰",    ")",    "Г",    "г",    "Ң",    "ң",    "Æ",    "æ",    "7",    "¾", },
-            { "I",        "i",    "|",    "\\",    "Ш",    "ш",    "Ғ",    "ғ",    "Ü",    "ü",    "8",    "©", },
-            { "O",        "o",    "?",    "/",    "Щ",    "щ",    "Х",    "х",    "È",    "è",    "9",    "®", },
+            { _U_,        _u_,    "‰",    ")",    "Г",    "г",    "Ң",    "ң",    "Æ",    "æ",    "7",    "¾", },
+            { _I_,        _i_,    "|",   "\\",    "Ш",    "ш",    "Ғ",    "ғ",    "Ü",    "ü",    "8",    "©", },
+            { _O_,        _o_,    "?",    "/",    "Щ",    "щ",    "Х",    "х",    "È",    "è",    "9",    "®", },
             { "P",        "p",    "~",    "`",    "З",    "з",    "Ъ",    "ъ",    "É",    "é",    "0",    "™", },
         },
         -- second row

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

@@ -91,6 +91,7 @@ return {
         southwest = "Ė",
         "Ē",
         "Ě",
+        "ɛ",
     },
     _e_ = {
         "e",
@@ -104,6 +105,53 @@ return {
         southwest = "ė",
         "ē",
         "ě",
+        "ɛ",
+    },
+    _I_ = {
+        "I",
+        north = "Í",
+        northeast = "Í",
+        northwest = "Ì",
+        east = "Î",
+        west = "Ĩ",
+        south = "Į",
+        "Ī",
+        "ɪ", -- small capital letter i, near-close front unrounded vowel IPA
+    },
+    _i_ = {
+        "i",
+        north = "ï",
+        northeast = "í",
+        northwest = "ì",
+        east = "î",
+        west = "ĩ",
+        south = "į",
+        "ī",
+        "ɪ", -- small capital letter i, near-close front unrounded vowel IPA
+    },
+    _O_ = {
+        "O",
+        north = "Ó",
+        northeast = "Ó",
+        northwest = "Ò",
+        east = "Ô",
+        west = "Õ",
+        south = "Ǫ",
+        "Ō",
+        "ɔ", -- open o, open-mid back rounded vowel IPA
+        "ɒ", -- turned alpha, open back rounded vowel IPA
+    },
+    _o_ = {
+        "o",
+        north = "ö",
+        northeast = "ó",
+        northwest = "ò",
+        east = "ô",
+        west = "õ",
+        south = "ǫ",
+        "ō",
+        "ɔ", -- open o, open-mid back rounded vowel IPA
+        "ɒ", -- turned alpha, open back rounded vowel IPA
     },
     _S_ = {
         "S",
@@ -159,4 +207,28 @@ return {
         "Τ",
         "τ",
     },
+    _U_ = {
+        "U",
+        north = "Ü",
+        northeast = "Ú",
+        northwest = "Ù",
+        east = "Û",
+        west = "Ũ",
+        south = "Ų",
+        southwest = "Ů",
+        "Ū",
+        "ʌ", -- turned v, open-mid back unrounded vowel IPA
+    },
+    _u_ = {
+        "u",
+        north = "ü",
+        northeast = "ú",
+        northwest = "ù",
+        east = "û",
+        west = "ũ",
+        south = "ų",
+        southwest = "ů",
+        "ū",
+        "ʌ", -- turned v, open-mid back unrounded vowel IPA
+    },
 }