[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