Fix logic for notebook outline data source isEmpty() fn

[Yoyokrazy]

Fixes: #208553

Slightly different fix than expected. The notebook outline datasource will send all entries to the view provider, and then the viewprovider will filter out entries based on the relevant notebook outline filters.

However, the placeholder message referenced by this issue is controlled by the outline pane checking the outline's isEmpty fn. This calls down to the data source, which just checks if there is a nonzero number of entries.

Issue is that it doesn't take into account that entries do exist, but may be filtered out by the view provider.

Fix is to just bulk up the logic for isEmpty, checking the entries against the outline filters to take into account what the user will see vs the entries object itself.

cc/ @rebornix, previously we discussed keeping the outline filters out of certain layers. We had one of the settings here already, as it related to computing symbols. This pulls in the other two, but still keeps the filtering concept out of the data source itself which is what we initially tried to avoid.

Fixed behavior:

20241218-2142-48.4952111.mp4