Fix clippy::type_complexity, refactor type alias

Use a type alias to refactor the submit_funcs type definition, as a
vector of boxed SubmitFuncs. Note this type alias isn't used in the
parameterized add_submit_func() function because it accepts a trait, not
a type - no `dyn` keyword in this context, but it is needed in the type
definition.

src/ui/mod.rs

@@ -1494,6 +1494,8 @@ impl Button {
     }
 }
 
+type SubmitFunc = dyn Fn(&mut TextBox, &mut crate::Game);
+
 element! {
     ref TextBoxRef
     pub struct TextBox {
@@ -1505,7 +1507,7 @@ element! {
         priv text: Option<TextRef>,
         priv was_focused: bool,
         priv cursor_tick: f64,
-        priv submit_funcs: Vec<Box<dyn Fn(&mut TextBox, &mut crate::Game)>>,
+        priv submit_funcs: Vec<Box<SubmitFunc>>,
     }
     builder TextBoxBuilder {
         hardcode button = None,