core: Add clang-tidy (#8664)

This adds a .clang-tidy file for us.

It's not a strict requirement to be compliant, but I tuned it to be alright.

src/desktop/Window.cpp

@@ -443,7 +443,7 @@ void CWindow::moveToWorkspace(PHLWORKSPACE pWorkspace) {
     }
 }
 
-PHLWINDOW CWindow::X11TransientFor() {
+PHLWINDOW CWindow::x11TransientFor() {
     if (!m_pXWaylandSurface || !m_pXWaylandSurface->parent)
         return nullptr;