Polish seed UI options

Fix webui.sh not working

.gitignore - Exclude checkpoint files in models folder

Random button sets seed to -1, reuse copies the seed from the last
generated image.

change () to "" in .sh scripts because export AAA=(BBB CCC) sets AAA to BBB and does not export anything
whoever put that in .sh files - shame on you

Add some error handling for VRAM monitor

in case someone sets opts.memmon_poll_rate to 0 during generation

fix pasting of images from the clipboard in img2img (fixes #645)

altered progressbar to not rely on first progress request coming after the job has started; may help with broken progressbar some people say they have

split masking helper function from processing.py into new masking.py

fixed typos in processing.py