Fix Fill Tool color label combobox being enabled

Before this commit, color label combobox in Fill Tool would be
enabled in some specific cases, for example after disabling the
Fast Mode, even though the sampling method would be Current Layer
or All Layers. This commit fixes that behaviour by making sure
that the combobox is enabled only if all three are true:

• Fill Tool doesn't use Fast Mode
• Fill Tool doesn't fill the entire selection
• Fill Tool uses the Color Labeled Layers