Allow using extended regular expressions in the find dialogs

This adds support for searching "this|that" to find either "this" or "that"
or using quantifiers like "{number}". The change applies to the
Find/Replace dialog and the search bar.

See https://en.cppreference.com/w/cpp/regex/ecmascript for details of the
syntax.

This raises the requirement for an external version of QScintilla to v2.11.

See issue #1625

src/ExtendedScintilla.cpp

@@ -244,7 +244,8 @@ bool ExtendedScintilla::findText(QString text, bool regexp, bool caseSensitive,
     }
 
     return findFirst(text, regexp, caseSensitive, words, wrap, forward,
-                     /* line */ -1, /* index */ -1, /* show */ true, /* posix */ true);
+                     /* line */ -1, /* index */ -1,
+                     /* show */ true, /* posix */ true, /* cxx11 */ true);
 }
 
 void ExtendedScintilla::setEnabledFindDialog(bool value)

src/FindReplaceDialog.cpp

@@ -54,7 +54,8 @@ bool FindReplaceDialog::findFirst(bool wrap, bool forward)
              ui->wholeWordsCheckBox->isChecked(),
              forward,
              /* show */ true,
-             /* posix */ true);
+             /* posix */ true,
+             /* cxx11 */ true);
     else
         return m_scintilla->findFirst
             (ui->findText->text(),
@@ -66,7 +67,8 @@ bool FindReplaceDialog::findFirst(bool wrap, bool forward)
              /* line */ -1,
              /* index */ -1,
              /* show */ true,
-             /* posix */ true);
+             /* posix */ true,
+             /* cxx11 */ true);
 }
 
 bool FindReplaceDialog::findNext()