TOC Widget: Improve mapping between target header and highlighted entry.

Center targeted content in rough middle of screen rather than top. This
is more consistent with everything else and scales better across
large/small files and large/small displays.
Avoid polluting history when clicking entries.
Do not give keyboard focus to table of content entries.
Make font slightly smaller.
Symbols View Widget: Fix issue where long entries took up more than 1 line.
Symbols View Widget: Dismiss on double click.

Improve scroll to destination (very minor, for consistency).