Handle unordered add / position change events

Fixes (some cases of) plugins ignoring saved `(x, y)` coordinates when a
project is loaded. The canvas can get notified of a plugin's position
before the plugin is actually added, so when the plugin is actually it
just goes to the default position.

This happens because `PatchbayClientPositionChanged` events can arrive
before `PatchbayClientAdded` events, as seen in debug logs like this:

```
...
PatchCanvas::joinGroup(30)
PatchCanvas::joinGroup(30) - unable to find groups to join
PatchCanvas::setGroupPos(30, 1118, 591, 0, 0)
PatchCanvas::setGroupPos(30, 1118, 591, 0, 0) - unable to find group to reposition
PatchCanvas::addGroup(30, b'Audio to CV', SPLIT_UNDEF, ICON_PLUGIN)
...
```

If this happens, the canvas box will be placed in the default position,
rather than obeying the `PatchbayClientPositionChanged` signal. This PR
keeps track of missed events and then applies them once the object is
actually added.

source/frontend/patchcanvas/__init__.py

@@ -145,6 +145,9 @@ class Canvas(object):
         self.initial_pos = QPointF(0, 0)
         self.size_rect = QRectF()
 
+        self.pending_group_joins = set()
+        self.pending_position_changes = {}
+
     def callback(self, action, value1, value2, value_str):
         print("Canvas::callback({}, {}, {}, {})".format(action, value1, value2, value_str))
 

source/frontend/patchcanvas/patchcanvas.py

@@ -400,6 +400,17 @@ def addGroup(group_id, group_name, split=SPLIT_UNDEF, icon=ICON_APPLICATION):
 
     canvas.group_list.append(group_dict)
 
+    # Callbacks can arrive out of order, so we might have pending events.
+    if group_id in canvas.pending_group_joins:
+        joinGroup(group_id)
+        canvas.pending_group_joins.remove(group_id)
+
+    if group_id in canvas.pending_position_changes:
+        position_change = canvas.pending_position_changes[group_id]
+        setGroupPosFull(*position_change)
+        del canvas.pending_position_changes[group_id]
+
+
     if options.eyecandy == EYECANDY_FULL and not options.auto_hide_groups:
         CanvasItemFX(group_box, True, False)
     else:
@@ -602,6 +613,7 @@ def joinGroup(group_id):
     # FIXME
     if not (item and s_item):
         qCritical("PatchCanvas::joinGroup(%i) - unable to find groups to join" % group_id)
+        canvas.pending_group_joins.add(group_id)
         return
 
     port_list_ids = list(item.getPortList())
@@ -755,6 +767,7 @@ def setGroupPosFull(group_id, group_pos_x_o, group_pos_y_o, group_pos_x_i, group
 
     qCritical("PatchCanvas::setGroupPos(%i, %i, %i, %i, %i) - unable to find group to reposition" % (
               group_id, group_pos_x_o, group_pos_y_o, group_pos_x_i, group_pos_y_i))
+    canvas.pending_position_changes[group_id] = (group_id, group_pos_x_o, group_pos_y_o, group_pos_x_i, group_pos_y_i)
 
 # ------------------------------------------------------------------------------------------------------------