feat(settings): persist the theme through the settings binding

The palette lives in Lua, so C++ stores the name only and ui.setTheme()
writes through here before rebuilding and repainting.

lua/test/ui.lua

@@ -17,6 +17,17 @@ fs = {
   end,
 }
 
+local savedTheme = "light"
+settings = {
+  getTheme = function()
+    return savedTheme
+  end,
+  setTheme = function(name)
+    savedTheme = name
+    return true
+  end,
+}
+
 local frameWidth, frameHeight = 320, 480
 
 gui = {
@@ -166,7 +177,7 @@ assert(table.concat(ui.themeNames(), ",") == "dark,light,mono")
 local ok, err = ui.setTheme "missing"
 assert(ok == nil and err == "Unknown theme")
 assert(ui.setTheme "dark" == true)
-assert(files["/.lua/theme"] == "dark" and ui.getTheme() == "dark")
+assert(savedTheme == "dark" and ui.getTheme() == "dark")
 
 local events = {}
 local function handler(name)