limit extension preload scripts to just one file named preload.py

do not mess with components' visibility for scripts; instead create group components and show/hide those; this will break scripts that create invisible components and rely on UI but the earlier i make this change the better

fix broken Restart Gradio button

added support for extensions

This reverts commit e2930f98.

SD upscale moved to scripts
Batch processing script removed
Batch processing added to main img2img and now works with scripts
img2img page UI reworked to use tabs

fixes bug #464, which happens when controls is None, which leads to args_to being None.