tdf#130857 qt weld: Handle special value -1 for cursor pos

Implement special handling for cursor position value
of -1 according to the comment above weld::Entry::set_position
in include/vcl/weld.hxx:

    // nCursorPos can be -1 to set to the end

Change-Id: I078d2123f391cf1fcab6f40d26309e2a2eb2315f
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/177448
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Tested-by: Jenkins

vcl/qt5/QtInstanceEntry.cxx

@@ -89,6 +89,9 @@ void QtInstanceEntry::replace_selection(const OUString& rText)
 void QtInstanceEntry::set_position(int nCursorPos)
 {
     SolarMutexGuard g;
+    if (nCursorPos == -1)
+        nCursorPos = m_pLineEdit->text().length();
+
     GetQtInstance().RunInMainThread([&] { m_pLineEdit->setCursorPosition(nCursorPos); });
 }