Stitching_Grid: fix ROIs when computing overlap

Updated the generateRois method of Stitching_Grid to properly generate
ROI coordinates for datasets stitched using the "compute overlap"
functionality.

Previously, the stage coordinates from metadata were used exclusively,
which could result in improper ROI coverage of an image stitched with
overlap computed.

Now, the actual translation values from the 0,0 position are used
instead, which works in determining tile positions whether or not
compute overlap was used.
Stitching_Grid: save ROIs when writing fused image

Previously, when fusing and writing to disk, ROIs were simply discarded.

The RoiManager has a command to write Rois to disk, however. So the
generateRois method and invocation were refactored to writing the final
ROIs to disk (in a "tile_rois.zip" in the output directory).

These saved ROIs can be applied after opening the saved dataset, by
opening the RoiManager and running "More >> Open...".
Merge branch 'compute_overlap_roi_fix'

Fixes a bug when generating ROIs for datasets fused with "Compute Overlap"
enabled.
Merge branch 'stitching_save_rois'

Enables writing ROIs to disk when the fused dataset is also saved to disk.
Stitching_Grid: add downsample option

The Stitching_Grid plugin now provides the option to downsample tiles
before stitching.

Effectively, this runs the "Image > Scale..." plugin on all tile images
before any stitching/fusion/etc... operations take place.

This functionality can be used to significantly reduce the size of a
dataset, e.g. when the resultant stitched image will not fit in memory.
Merge branch 'stitching_downsample'

Adds a "Downsample tile" option to the Stitching_Grid plugin, allowing for
downsampling of tiles before they are fused (significantly reducing the overall
dataset size, reducing memory and time requirements).