Up to 512px thumb files and huge icons size option

Hi

I added some options around thumbnails.

- Disk cache size of thumbs, normal 128px, large 256px and huge 512px.
  So now the huge zoom works well,
  https://github.com/jun7/rox-filer/wiki/Screenshots
  I don't think someone will use 512px though.

- Purge time of memory cache of thumbs. It was 1200sec. now 1 - 999999sec.

- Huge Icons size. 96 - 1024px. It is in "Display" page of the Options.


512px is useless though huge Icons size and 256px thumb files is usefull I think.

My fork is here:
https://github.com/jun7/rox-filer

Thanks.


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