tethering: filename input box + check before shot

This fixes the name of one setting (appears to be a hasty copy/paste), "capture/camera/storage/namepattern" is unused.

It also checks that the file pattern for captured images read from darktablerc actually contains $(SEQUENCE), and if not resets it to default (and writes that to darktablerc). For some reason I had a older/messed up darktablerc where the pattern didn't contain this, so that tethering would lock up as soon as I tried to pull more than 1 picture from my camera, since the sequence couldn't be incremented.

Probably it should be better to warn the user but I'm not sure how to do that.