Blender Git Loki

UI: Add shortcuts for constraint panels

Only the delete shortcut applies here, although the move up and down
operators can optionally be assigned in the keymap.

See rB1fa40c9f8a81 for more details and rB5d2005cbb54b for the
grease pencil modifier panel implementation, which is the same.
Some refactoring of the constraint delete operator was necessary,
including adding an invoke function.

Differential Revision: https://developer.blender.org/D8238