Factor out theme hook suggestion building from ThemeManager::render() into a separate function.

I am proposing a straightforward refactoring.

Currently, theme hook suggestions are collected within ThemeManager::render().
I propose to split this out into a separate function.

Let's start with a private function within the same class. Later we can decide to move this elsewhere, make it protected, or whatever else.
Let's really keep it private for now to allow future refactoring.