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.