Use C functions for the backstack, instead of a lua module

Working with the default group and root kinda sucks if you have to do it
from lua!

lua/backstack.lua

@@ -1,37 +0,0 @@
-local lvgl = require("lvgl")
-
-local backstack = {
-  root = lvgl.Object(nil, {
-    w = lvgl.HOR_RES(),
-    h = lvgl.VER_RES(),
-  }),
-  stack = {},
-}
-
-function backstack:Top()
-  return self.stack[#self.stack]
-end
-
-function backstack:SetTopParent(parent)
-  local top = self:Top()
-  if top and top.root then
-    top.root:set_parent(parent)
-  end
-end
-
-function backstack:Push(screen)
-  self:SetTopParent(nil)
-  table.insert(self.stack, screen)
-  self:SetTopParent(self.root)
-end
-
-function backstack:Pop(num)
-  num = num or 1
-  for _ = 1, num do
-    local removed = table.remove(self.stack)
-    removed.root:delete()
-  end
-  self:SetTopParent(self.root)
-end
-
-return backstack

lua/main.lua

@@ -1,3 +1,4 @@
 local backstack = require("backstack")
-local main_menu = require("main_menu"):Create(backstack.root)
-backstack:Push(main_menu)
+local main_menu = require("main_menu")
+
+backstack.push(main_menu)

lua/main_menu.lua

@@ -3,11 +3,9 @@ local widgets = require("widgets")
 local legacy_ui = require("legacy_ui")
 local database = require("database")
 
-local main_menu = {}
-
-function main_menu:Create(parent)
+return function()
   local menu = {}
-  menu.root = lvgl.Object(parent, {
+  menu.root = lvgl.Object(nil, {
     flex = {
       flex_direction = "column",
       flex_wrap = "wrap",
@@ -46,5 +44,3 @@ function main_menu:Create(parent)
 
   return menu
 end
-
-return main_menu