Cleanup: Rename & refactor several F-curve functions

Rename and refactor several F-curve key manipulation functions, and move
them from `editors` to `blenkernel`.

The functions formerly known as `delete_fcurve_key`,
`delete_fcurve_keys`, and `clear_fcurve_keys` have been moved from
`ED_keyframes_edit.h` to `BKE_fcurve.h` and have been renamed according
to hierarchical naming rules.

Below is a table of the naming changes.

| From | To |
| -- | -- |
| `delete_fcurve_key(fcu, index, do_recalc)` | `BKE_fcurve_delete_key(fcu, index)` |
| `delete_fcurve_keys(fcu)` | `BKE_fcurve_delete_keys_selected(fcu)` |
| `clear_fcurve_keys(fcu)` | `BKE_fcurve_delete_keys_all(fcu)` |
| `calchandles_fcurve()` | `BKE_fcurve_handles_recalc()` |
| `calchandles_fcurve_ex()`| `BKE_fcurve_handles_recalc_ex()` |

The function formerly known as `delete_fcurve_key` no longer takes a
`do_fast` parameter, which determined whether or not to call
`calchandles_fcurve`. Now, the responsibility is on the caller to run
the new `BKE_fcurve_handles_recalc` function if they have want to
recalculate the handles.

In addition, there is now a new static private function called
`fcurve_bezt_free` which sets the key count to zero and frees the key
array. This function is now used in couple of instances of functionally
equivalent code. Note that `BKE_fcurve_delete_keys_all` is just a
wrapper around `fcurve_bezt_free`.

This change was initially spurred by the fact that `delete_fcurve_keys`
was improperly named; this was a good opportunity to fix the location
and naming of a few of these functions.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D15282

source/blender/python/intern/bpy_rna_anim.c

@@ -487,7 +487,8 @@ PyObject *pyrna_struct_keyframe_delete(BPy_StructRNA *self, PyObject *args, PyOb
         i = BKE_fcurve_bezt_binarysearch_index(fcu->bezt, cfra, fcu->totvert, &found);
         if (found) {
           /* delete the key at the index (will sanity check + do recalc afterwards) */
-          delete_fcurve_key(fcu, i, 1);
+          BKE_fcurve_delete_key(fcu, i);
+          BKE_fcurve_handles_recalc(fcu);
           result = true;
         }
       }