E-book viewer: Fix searching for short strings in text with lots of similar entries displaying incorrect matches. Fixes #1931566 [some bad search results using regex](https://bugs.launchpad.net/calibre/+bug/1931566)

Up the amount of context used for matches from 10 to 30 chars to
reduce probability of false matching.

src/calibre/gui2/viewer/search.py

@@ -691,7 +691,7 @@ class SearchPanel(QWidget):  # {{{
                 try:
                     for i, result in enumerate(search_in_name(name, search_query)):
                         before, text, after, offset = result
-                        q = (before or '')[-5:] + text + (after or '')[:5]
+                        q = (before or '')[-15:] + text + (after or '')[:15]
                         result_num += 1
                         self.results_found.emit(SearchResult(search_query, before, text, after, q, name, idx, counter[q], offset, result_num))
                         counter[q] += 1

src/pyj/read_book/find.pyj

@@ -153,11 +153,11 @@ def select_search_result(sr):
     q = ''
     before_len = after_len = 0
     if sr.before:
-        q = sr.before[-5:]
+        q = sr.before[-15:]
         before_len = q.length
     q += sr.text
     if sr.after:
-        after = sr.after[:5]
+        after = sr.after[:15]
         after_len = after.length
         q += after
     match = find_specific_occurrence(q, int(sr.index), before_len, after_len, cache.text_map, sr.from_offset)

src/pyj/read_book/search_worker.pyj

@@ -168,7 +168,7 @@ def search_in_text_of(name):
         start, end = m.index, r.lastIndex
         before = haystack[Math.max(0, start - ctx_size):start]
         after = haystack[end:end+ctx_size]
-        q = (before or '')[-5:] + text + (after or '')[:5]
+        q = (before or '')[-15:] + text + (after or '')[:15]
         match_counts[q] = match_counts[q] or 0
         wc.result_num += 1
         result = {