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Animation: Snap Cursor Value operator

Add operator to snap the 2D Cursor value to selected keyframes. This is
doing almost the same as the "Cursor to Selected" operator, except that
it doesn't affect the current frame, just the Y-coordinate (the value)
of the 2D cursor.

The "snap cursor" operators are added to the Key ? Snap menu and to the
Snap pie menu. This means that these menus are now extended in meaning,
to not only mean "snap the selected keyframes to the cursor", but also
for some options "snap the cursor to selected keyframes".

This fixes T76596.