Port the search interface from QRegExp to QRegularExpression

• Remove the kateregexp class:

  • Move isMultiLine() and repairPattern() to kateregexpsearch; merge them into one function
  • Dot '.' character will match any character except a newline by default

• Explicitly enable QRegularExpression::MultilineOption, for the rationale see the comment at the top of KateRegExpSearch::search()

• KateCommands::SedReplace::InteractiveSedReplacer::InteractiveSedReplacer() was updated to use QRegularExpression, but not tested

• Adjust the extended context menu used in the find box:

  • QRegularExpression supports positive/negative fixed-length lookbehind
  • \x.... escape sequences must be wrapped in curly braces if more than two hex digits is used, so it's better to just always wrap \x escape sequencess in curly braces (QRegularExpression upstream recommendation), c.f.:
    • https://phabricator.kde.org/D26123#inline-147157
    • https://perldoc.perl.org/perlre.html

• Tweak/extend the relevant unit tests (searchbar_test, regexpsearch_test)

• All unit tests pass except for these ones (which fail on master too): kateindenttest_testCppstyle (Child aborted) kateindenttest_testPascal (Child aborted) bug313759 (Child aborted)