improve performance of 3-way merge on machines with not enough ram, by only accessing two of the models at a time

Now only traverse images when directory exists

Problem
everytime I modified pt files in embedding_dir or hypernetwork_dir, I
need to restart webui to have the new files shown in the dropdown of
Train Tab

Solution
refactored create_refresh_button out of create_setting_component so we
can use this method to create button next to gr.Dropdowns of embedding
name and hypernetworks

Extra Modification
hypernetwork pt are now sorted in alphabetic order

Default value is the first value of selectables.
Particually, None in styles.

made save style button not die when you cancel

This is only necessary when you have to train,
and compiling model produces warning.