Added possibility to set the theme (light or dark)

Launch the UI in dark mode with the --dark-mode switch

also useful for keeping settings cache and restore logic together, and nice for code reuse (other third party scripts can import this class)

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

left / top alignment was necessary with gradio 3.4.1. In gradio 3.5 the parent div of the image mask is centered, so the left / top alignment put the mask in the wrong place as described in #2750 #2795 #2805. This fix was tested on Windows 10 / Chrome.

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

revert changes to two bat files I asked to revert but the author couldn't in 863e9efc.

* moved repository url to changeable environment variable

* move stable diffusion repo itself to env

* added missing env

* Remove default URL
Co-authored-by: AUTOMATIC1111 <16777216c@gmail.com>

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