This implements 2 ideas I evoked in a blog post comment, concerning the usability of the syntax definition priority GUI:
- provide contextual help in a tooltip. I had never discovered the "what's this" property is set on the priority spinbox because that is something not made evident by the UI. Some usage feedback is useful here IMHO because the effect of a higher priority value isn't necessarily apparent directly.
- allow negative priority values. This requires no other changes (in the syntax-highhlighting FW) and makes it possible to "filter out" individual definitions (file types) that aren't (won't ever be) relevant and that can conflict with multiple other types. Think of a file type that uses a very common extension used by a number of other types that are more relevant. Decreasing the priority of a single type is less cumbersome than increasing the priority of a possibly unknown number of other types.
