Add basic search functionality to debug view dialog

Expected behavior:

• The search text is applied immediately, jumping the text edit to the relevant position and selecting the relevant text
• Search is plain text, not regex
• When pressing enter, the next item is selected
• Previous / next button to get previous / next result; search wraps when reaching beginning / end

(cherry picked from commit 0b5a2523e9aa1df87ad78fe3e471565d82f1c47f)

• GnuPG-Bug-ID: T7552