Issue #10026: Fix resizing issue with layer attributes checkboxes

This patch attempts to resolve #10026 (closed).

Unlike in 2.10, checking a GtkCheckButton makes its label bold. The increase in the font-weight also makes the label a bit wider. Normally this is fine, but if the label is the widest item in the GtkBox, it causes the box to expand and contract slightly. As the reporter noted, one case of this is the Lock position and size checkbutton in the Layer and Channel Attributes dialogue.

The proposed solution is to enclose the labels in a GtkScrolledWindow with its horizontal scrollbar policy set to GTK_POLICY_EXTERNAL. This allows us to set its initial size to match the width of the checkbuttons without it growing when they are checked/unchecked.

This seems to work, but may be a bit overkill. Feedback appreciated.