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_mods.lua

@@ -163,7 +163,7 @@ local function handle_buttons(tabview, fields, tabname, tabdata)
 end
 
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-tab_mods = {
+return {
 	name = "mods",
 	caption = fgettext("Mods"),
 	cbf_formspec = get_formspec,