Summary: This patch fixes an issue where, when switching between presets in the "Edit brush settings" panel, the Isotropic Spacing and Update Between Dabs features of the Spacing option would not get loaded from the newly selected preset.

Test Plan:
The following steps show the difference in behavior.
1. Open the "Edit brush settings" panel and select a preset from a brush engine that has the Spacing option (e.g. Airbrush_linear).
2. Check the Isotropic Spacing checkbox in the Spacing option. (The Spacing option itself doesn't have to be checked.)
3. Select a different preset (from the same brush engine) that does not use isotropic spacing (e.g. Airbrush_pressure).
4. Since the newly selected preset does not use isotropic spacing, the expected behavior is that the Isotropic Spacing checkbox becomes unchecked. Without the patch, it instead stays checked.
Similar behavior occurs with the Update Between Dabs checkbox. Checkboxes in other curve options, such as Mirror and Scatter, seem to work correctly with or without the patch.

This patch causes no new unit test failures on the author's machine.

Reviewers: #krita, rempt

Reviewed By: #krita, rempt

Subscribers: allenmarshall

Differential Revision: https://phabricator.kde.org/D6461