change working directory for WebMCam.exe frame .png writes

[Starky2]

Would be nice for SSD users to minimize writes to C:/%appdata%local/temp by being able to specify a different directory for the .png frames to be written

[dannyblank]

Having a "Clear WebMCache" button or "Clear WebMCache on record" Tickbox in the options to clear cache would be a neat way of keeping the folder size down too. (Sometimes it takes up gigs of space)