Selecting a block showed one grip, at its insertion point, so there was
no handle to pick an attribute up by: an attribute could only be moved
by moving the whole block.
Give the INSERT one grip per attribute, taken from the attribute's own
`Grippable` implementation, at ids `1 + index`. A drag on one moves that
attribute alone, through the same `apply_grip` the attribute already has
(which carries `alignment_point` along, so non-left alignments stay put
relative to the text). Grip 0 keeps translating the block and every
attribute with it (#255).
A constant attribute belongs to the block definition and an invisible one
is not drawn, so neither offers a grip; a locked-position attribute
declines on its own.
FixesHakanSeven12/OpenCADStudio#1259
Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01TQzYZsXBwPZ5HwBaGitEUV
(cherry picked from commit 7eb966f6a2)