FindReplaceOverlay: improve error indication for find and replace inputs

[HeikoKlare]

The find and replace input strings of the FindReplaceOverlay are temporarily colored red to indicate failures of find or replace operations, such as no matches being found for the input string or regular expressions being invalid. The behavior is currently inconsistent and partly unexpected. For example:

• The field to be colored is selected by the one having focus rather than the one conforming to the current operation; so a failing replace operation will color the search input field if that one has focus
• Whenever the replace input field shall be colored, the search input field is also colored
• When a find operation following a successful replace fails (because there are not more matches), the replace field is erroneously colored

With this change, the error feedback via coloring the find and replace input strings is streamlined based on the two following goals:

• Only one of the input fields is colored at one point in time
• Which of the fields is colored depends on whether a find or a replace operation failed
• The coloring is cleared iff either any of the two input fields loses focus or another find/replace operation was performed

[github-actions]

Test Results

 1 818 files  ±0   1 818 suites  ±0   1h 41m 17s ⏱️ + 9m 16s
 7 733 tests ±0   7 505 ✅ ±0  228 💤 ±0  0 ❌ ±0 
24 360 runs  ±0  23 611 ✅ ±0  749 💤 ±0  0 ❌ ±0 

Results for commit 2524500. ± Comparison against base commit 8681223.

♻️ This comment has been updated with latest results.