Don't eat colons in booru tags

Added dark mode switch

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.