[KMenuEdit] Port QRegExp to QRegularExpression

Test Plan:
- Start kmenuedit, create a new sub-menu somewhere, name it t-1
- Creat a new submenu in the same location and name it t-1 again
  the submenu is created but named t-2 automatically

- Copy an existing item to the same location from where you copied it,
  note the "-number" suffix that's added
- Drag and drop a .desktop file twice on the same location, note the
  suffix that's added

- The createDesktopFile() part, in tree.cpp, strips the name to a base
  that's then fed to KService::newServicePath(), e.g. "foo-1.desktop" to
  "foo"

Reviewers: #plasma, davidedmundson, mlaurent, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28519