Add margins to System Activity window

Right now it is deliberately setting the margins to zero, which causes
all of its UI controls and framed areas to touch the window border,
which looks bad and is just an incorrect thing to do in general for a
thing that does not deliberately provide its own margins for content.

Let's not do that anymore, so the window looks better.

BUG: 426661
FIXED-IN: 5.20

systemmonitor/ksystemactivitydialog.cpp

@@ -44,8 +44,6 @@ KSystemActivityDialog::KSystemActivityDialog(QWidget *parent)
     m_processList.setScriptingEnabled(true);
     setSizeGripEnabled(true);
 
-    layout()->setContentsMargins(0,0,0,0);
-
     // Since we kinda act like an application more than just a Window, map the usual ctrl+Q shortcut to close as well
     QAction *closeWindow = new QAction(this);
     closeWindow->setShortcut(QKeySequence::Quit);