Currently, all headers in CTabFolders without an image have a large text
padding. This applies to both the case where no image is assigned to a
tab item and the case where images shall not be shown as the properties
showSelectedImage and showUnselectedImage are both false.
While the latter is intended behavior to switch between image-based tab
folders and image-less, padding-based tab folders (see #785), the former
unintentionally breaks with the existing UI experience for CTabFolders
that do not use images.

This change makes the identification of whether large text padding shall
be applied explicit by factoring it out into a method (which may be
replaced by a different implementation later on). It reduces the cases
in which the large text padding is applied to the intended case
(showSelectedImage = showUnselectedImage = false) and restores the
behavior for all existing use cases.

Contributes to
https://github.com/eclipse-platform/eclipse.platform.swt/issues/945