Various search improvements

- Do not automatically reset the search start line on search hits
This was causing the annoying behavior that if you were to pause while typing
in a search term, and it would actually find a hit, then you would be searching
for the term a second time once you finish typing.

This commit introduces two new behaviors:
1. When you show the search bar, all searching will be done from the first
visible line in the terminal.
2. This start position is only reset when you
advance to the next result, by pressing "next", "previous", or the shortcuts
RETURN or SHIFT-RETURN

- When the view scrolls to a result, it will appear centered.
- When showing the search bar, do not invoke a search, only apply the highlight filters
- Add "search from beginning/end" button
This will scroll to the top/bottom before searching
Ctrl+return is also bound to this action
- Text highlighted by mouse will be set as the current search text when the incremental
search bar is opened
- Add "Search backwards" to search bar options
Button text is swapped according to reverse search switch