Creating TileLayers in groups via API do not inherit the TileMaps size

[Miepee]

Describe the bug
If a tileLayer that's getting added to a tileMap does not have a size set but tileMap does, then Tiled tries to set the layer's size tot he map size:

    // If this map has a valid size but the tile layer that's getting added
    // doesn't, default the layer's size to the map size.
    if (editableLayer->isTileLayer()) {
        auto editableTileLayer = static_cast<EditableTileLayer*>(editableLayer);
        if (editableTileLayer->size().isNull() && !size().isNull())
            editableTileLayer->setSize(size());
    }

This however does not occur if the tilerLayer that's being added is part of a group layer.

To Reproduce
Via the js API, do the following:

• create a tilemap with a certain size
• create a groupLayer
• create a tileLayer and add it to the groupLayer

Expected behavior
The tileLayer that's being added to the group should get the size set to the maps' size if it has a valid one.

Platform (please complete the following information):

• OS: Windows 10
• Version: Tiled 1.8.2