Related fdo#78323 GtkFrame has additional padding

Comparing the three tab pages, shows that the preview field at the bottom is different:

Line: GtkBox
Line Styles and Arrow Styles: GtkFrame

The preview field is exactly 2 px smaller for the GtkFrame tab pages, compared to GtkBox tab page.

This patch fixes this. Now all tab pages look the same.

Change-Id: Ia94bda6c37fee3c8864f8351e157b2026a735361
Reviewed-on: https://gerrit.libreoffice.org/9283
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>