Fix tool-system index error with mode-specific tools added to groups

It was possible to have invalid indices when a script registered
a tool into an existing group of tools.

Address issue raised by D7436

Fix T81155: Keyframe.select_control_point True when unselected

Check the selection flag since BEZT_FLAG_TEMP_TAG might be set.

Cleanup: spelling

Cleanup: remove 'r_' prefix for variables

This is for return arguments, also remove redundant NULL check.

Keymap: use release event instead of click

Minor change to last commit, prefer release as click depends on
not moving the mouse cursor.

Keymap: use D-key click to avoid conflict with grease pencil drawing

Fix T73858: Gizmo.use_draw_scale doesn't work as expected

Fix T68024: Crash from missing original-index mesh layer

Constructive modifiers were not initializing an original index layer
in the case a previous deform modifier created 'mesh_final'.

This happened in the case of multiple deform modifiers that deform along
normals, as requesting normals caused the final mesh to be created.

Ensure mapping data is created in the case
only non-constructive modifiers have run.

Cleanup: re-order constructive modifier checks

Minor changes to simplify fix (coming next), no functional changes.

Cleanup: comments, spelling