[fix, spec] InputText:addChars() unicode handling (#3729)

Also rename from `addChar` to `addChars` for clarity.

Fixes #3703.

spec/unit/inputtext_spec.lua

@@ -0,0 +1,57 @@
+describe("InputText widget module", function()
+    local InputText
+    local equals
+    setup(function()
+        require("commonrequire")
+        InputText = require("ui/widget/inputtext")
+
+        -- thanks to https://stackoverflow.com/a/32660766/2470572
+        equals = function(o1, o2, ignore_mt)
+            if o1 == o2 then return true end
+            local o1Type = type(o1)
+            local o2Type = type(o2)
+            if o1Type ~= o2Type then return false end
+            if o1Type ~= 'table' then return false end
+
+            if not ignore_mt then
+                local mt1 = getmetatable(o1)
+                if mt1 and mt1.__eq then
+                    --compare using built in method
+                    return o1 == o2
+                end
+            end
+
+            local keySet = {}
+
+            for key1, value1 in pairs(o1) do
+                local value2 = o2[key1]
+                if value2 == nil or equals(value1, value2, ignore_mt) == false then
+                    return false
+                end
+                keySet[key1] = true
+            end
+
+            for key2, _ in pairs(o2) do
+                if not keySet[key2] then return false end
+            end
+            return true
+        end
+    end)
+
+    describe("addChars()", function()
+        it("should add regular text", function()
+            InputText:initTextBox("")
+            InputText:addChars("a")
+            assert.is_true( equals({"a"}, InputText.charlist) )
+            InputText:addChars("aa")
+            assert.is_true( equals({"a", "a", "a"}, InputText.charlist) )
+        end)
+        it("should add unicode text", function()
+            InputText:initTextBox("")
+            InputText:addChars("Л")
+            assert.is_true( equals({"Л"}, InputText.charlist) )
+            InputText:addChars("Луа")
+            assert.is_true( equals({"Л", "Л", "у", "а"}, InputText.charlist) )
+        end)
+    end)
+end)