Add dumb theme item cache to Control

[YuriSizov]

I've experimented with this a year ago, and in targeted testing it didn't yield any meaningful performance improvements. But there seems to be a benefit to it still in some extreme situations, like mentioned in #64241. So I'm reviving it, as it shouldn't have any downsides and would be beneficial for cases that abuse theme item fetching without a cache on the user side.

Out of curiosity, I've retested the snippet from #64271 (review) again, and got a huge boost, on top of the caching added by @KoBeWi in that PR. From ~5.4s down to ~4.3s in debug builds! There are clearly caches missing in some of the editor log controls. Likely, RichTextLabel is doing something nasty.

This can be backported to 3.x, adjusted for the code structure there.

cc @AaronRecord since you're interested in optimizing theme.

[jordo]

From ~5.4s down to ~4.3s in debug builds!

Our benchmarks in 3.5 here #64329 on purely the affected get_* control functions show much more than a 20% improvement. Seeing 600% from a quick and dirty gdscript test in editor, and up to 20X speedup when called natively on release export builds.