Change maximum possible border size

Due to larger images the maximum number of additional border pixel may be to small -> enlarge maximum value within slider.

Suggestion: Replace the slider with an other input method, e.g., spin box or any text field. Make sure that only integer input is allowed. Also remove the maximum boundary to be more flexible. Add the unit [2 px] for documentation.

assigned to @l.dressen

created branch 326-change-maximum-possible-border-size to address this issue

mentioned in merge request !225 (merged)

closed with merge request !225 (merged)

mentioned in commit 2d325add