In all themes, fix the color of selected text, while editing a
tree-view's item text (such as when renaming a layer), by
overriding tree-view specific styling with the global text-entry
style, for nested text entries inside tree-views.  The text would
previously use the same color as the selection background, making
it unreadable.

This issue affects the Dark and Gray themes, although this fix only
takes care of the Dark theme.