Draft: UX Issue #678: Add Merge Selected Paths action
This patch attempts to resolve Teams/GIMP/Design/gimp-ux#678
It adds a new "Merge Selected Paths" action to the Paths menu, which lets you merge together all paths that you have selected (whether visible or not).
@tilkau, @Stratelier Hi! I set this to build an AppImage if you want to try it out (I can make a Windows installer generate as well if you're on Windows).
@Jehan Hey! A question whenever you have time. Code-wise, I made a dedicated gimp_image_merge_selected_paths () function in gimpimage-merge.c. Would it be more flexible if I changed that to gimp_image_merge_paths (), which takes in a list of paths and tries to merge those? Then I could just pass the selected paths from the "merge selected paths" action, and potentially reuse that function elsewhere.