Clean up layouts and use minisplitter everywhere

This patch replaces all uses of QSplitter with thin 1-pixel
splitters. I also fixed dock widget layouts in main windows
to do the same. Additionally I had to tweak the tabbars so
they look good in mini splitter layouts to avoid a double
left-border.

Reviewed-by: thorbjorn

src/plugins/debugger/watchwindow.cpp

@@ -125,6 +125,7 @@ WatchWindow::WatchWindow(Type type, DebuggerManager *manager, QWidget *parent)
     m_grabbing = false;
 
     QAction *act = theDebuggerAction(UseAlternatingRowColors);
+    setFrameStyle(QFrame::NoFrame);
     setWindowTitle(tr("Locals and Watchers"));
     setAlternatingRowColors(act->isChecked());
     setIndentation(indentation() * 9/10);