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_settings.lua

@@ -384,7 +384,7 @@ local function handle_settings_buttons(this, fields, tabname, tabdata)
 	return ddhandled
 end
 
-tab_settings = {
+return {
 	name = "settings",
 	caption = fgettext("Settings"),
 	cbf_formspec = formspec,