add settings API

samples/settings-test.lua

@@ -0,0 +1,18 @@
+function on_resume()
+  local s1 = {
+    key1 = "value1",
+    key2 = "value2",
+  }
+
+  settings:set_kv(s1)
+
+  local s2 = settings:get_kv()
+
+  ui:show_text("key1="..s2.key1.." ".."key2="..s2.key2)
+end
+
+function on_settings()
+  local s = settings:get_kv()
+
+  ui:show_dialog("Settings", "key1="..s.key1.." ".."key2="..s.key2)
+end