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_credits.lua

@@ -69,7 +69,7 @@ local previous_contributors = {
 	"Zefram <zefram@fysh.org>",
 }
 
-tab_credits = {
+return {
 	name = "credits",
 	caption = fgettext("Credits"),
 	cbf_formspec = function(tabview, name, tabdata)