lv_cache_acquire_or_create() function

Acquire a cache entry with the given key. If the entry is not in the cache, it will create a new entry with the given key. If the entry is found, it's priority will be changed by the cache's policy. And the `lv_cache_entry_t::ref_cnt` will be incremented. If you want to use this API to simplify the code, you should provide a `lv_cache_ops_t::create_cb` that creates a new entry with the given key. This API is a combination of lv_cache_acquire() and lv_cache_add(). The effect is the same as calling lv_cache_acquire() and lv_cache_add() separately. And the internal impact on cache is also consistent with these two APIs.