Created attachment 331515 [details] [review]
editor-frame: Move the logic for enabling/disabling replace actions

There are many different conditions that determine whether Replace or
Replace All operations would make sense, so similar logic needs to be
run in many callbacks to try to ensure the buttons' sensitivity reflects
the state of the search. This commit centralizes that logic into
update_replace_actions_sensitivity() which makes the code more
maintainable.

Created attachment 331516 [details] [review]
editor-frame: Add squiggly underlines for invalid regex

When an invalid regex is entered in the search entry field, it's
difficult for the user to tell if there's an issue with their input or
if the search genuinely returned no results. This commit addresses that
ambiguity by adding a wavy underline when an invalid regular expression
is entered in the search field, or an invalid replacement string is
entered in the replace field (which can contain backreferences). The
color white is used so that it can still be seen when the background
is red (which happens when a search returns no results).

Review of attachment 331515 [details] [review]:

LGTM, nice work

Review of attachment 331516 [details] [review]:

Cool!

Pushed as commit 302278e573f667d6f10c67f09644a3f1a670f276 and commit e8d39eeabb9f940f2e5aa2003b8021470bfc6255