Mainmenu: Refactor tab UI code

- Use local variables for tabs in place of globals
 - Merge together if statements where possible
 - Replace manual table searching code with indexof where possible

builtin/mainmenu/tab_singleplayer.lua

@@ -241,10 +241,10 @@ local function on_change(type, old_tab, new_tab)
 end
 
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-tab_singleplayer = {
+return {
 	name = "singleplayer",
 	caption = fgettext("Singleplayer"),
 	cbf_formspec = get_formspec,
 	cbf_button_handler = main_button_handler,
 	on_change = on_change
-	}
+}