Paste as New Layer rather than Floating Selection

This patch makes a first step towards "less floating selection" in the Gimp 3.0 roadmap. It changes the default selection pasting behavior from creating a new floating selection to creating a new layer above. This is done by replacing the paste enums in edit-actions.c from GIMP_PASTE_TYPE_FLOATING to GIMP_PASTE_TYPE_NEW_LAYER. The "Paste as New Layer" options are also removed from the submenu since they are now redundant.