Export sprite sheet to document/tab instead of file

[KasumiArai]

For import sprite sheet, the current document is automatically the default, but one can select a file to import if one wants.

Export sprite sheet must create a file. Older versions of the program had an option to export only to the currently opened document. (Like the current "open generated sprite sheet" but without saving first) .I forget when exactly this went away, but I would like to see it back. Often I do a lot of sprite sheet importing and exporting to transform data (or for copying the result to paste in another file), and don't need a file created for every step.