auto-increment placeholder for the new worlds

res/layouts/pages/new_world.xml

@@ -2,7 +2,7 @@
     <label>@Name</label>
     <textbox id='name_box'
              validator='world_name_validator'
-             placeholder='New World' 
+             placeholder='-'
              padding='4'>
     </textbox>
     <label>@Seed</label>

res/layouts/pages/new_world.xml.lua

@@ -24,6 +24,16 @@ function create_world()
     core.new_world(name, seed, generator)
 end
 
+function world_name_placeholder()
+    local name = "New World"
+    local number = 0
+    while world.exists(name) do
+        number = number + 1
+        name = "New World " .. tostring(number)
+    end
+    return name
+end
+
 function on_open()
     document.content_btn.text = string.format(
         "%s [%s]", gui.str("Content", "menu"), #pack.get_installed()
@@ -38,6 +48,7 @@ function on_open()
         settings.generator_name(generation.get_generators()[settings.generator])
     )
     document.name_box.text = settings.name or ''
+    document.name_box.placeholder = world_name_placeholder()
     document.seed_box.text = settings.seed or ''
     document.seed_box.placeholder = tostring(math.random()):sub(3)
 end