Improve Find and Replace dialogs text entry background management

[ntrel]

Always reset backgrounds first on any dialog response, then set again if necessary.

• When showing a dialog the background was only reset when using the document selected text, but the document contents could have changed e.g. to contain the previously unmatched search text, yet the background remained set.
• The Find All responses didn't clear the background even when successful. This fixes those situations.

Set search entry background when there's a regex or escape error.
Set replace text entry background when there's an escape error.