app: save images with fractional grid coordinates as version-10 XCFs
Fractional-coordinate support for image grids was added in commit 1572bccc, right before the introduction of XCF version 10. While images with fractional grid coordinates can be loaded with earilier versions of GIMP, the grid coordinates are rounded to the nearest integer. Bump the minimal XCF version when saving images with fractional grid coordinates to 10, which should have been the case all along.
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