WIP: Multi-step undo port from pd vanilla

Partial port of the extended Pdvanilla undo system to PurrData

Ported:

g_undo.h

• Define enum 't_undo_type' for undo types
• Change t_undo_action 'type' attribute from int to 't_undo_type'

g_undo.c

• Join the four big switches inside canvas_undo_undo/redo/rebranch/free into a unique switch inside the new canvas_undo_doit function. Action to be performed is passed as a paramenter.
• Replace the four mentioned switch with the new function
• Add support for atomic (and nested) multi-step sequences in canvas_undo_undo and canvas_undo_redo using the new types SEQUENCE_START/END as checkpoints.

Not ported:

• New undo queue/attibutes container, 't_undo'
• New queue location at glist/gl_privatedata/undo (a t_undo element)
• Query and modification procedures for this new storage method/location
• Combined action+undo_add methods. Example: canvas_connect_with_undo, ...
• Special paste undo action constructor: canvas_undo_set_pastebinbuf
• Change canvas_undo_add 'type' paramenter from int to t_undo_type.

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Usage

canvas_undo_add(canvas, UNDO_SEQUENCE_START, "name", 0);
/* Basic undo actions */
canvas_undo_add(canvas, UNDO_SEQUENCE_END, "name", 0);