Grid align count fixes.

• Don't override the row/column counts when total object count isn't a perfect multiple of them
• Properly react to changes done using keyboard
• Don't override row/column when setting numbers with empty selection

Fixes #5380 (closed)

After the changes number input will only react to when editing is finished (enter is pressed, focus is changed,tab is pressed, ...). The signal that was connected previously happened after each keystroke, before it was processed by number input and converted to number. While in some other parts of Inkscape UI more quicker feedback might be desired, I would say in this case it's acceptable. Better than buggy reaction based on old input.

Potential future improvements for grid alignment tool (outside the scope of this bugfix merge request):

• tab order for rows/columns (should be row, column, checkbox, checkbox, not rows, checkbox, column, checkbox)
• grid is currently row based, left to right, top to bottom, would be nice to have option for other grid layout orders
• mode for setting anchor distance instead of spacing between bounding boxes