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/threadswindow.cpp

@@ -43,6 +43,7 @@ ThreadsWindow::ThreadsWindow(QWidget *parent)
 {
     QAction *act = theDebuggerAction(UseAlternatingRowColors);
 
+    setFrameStyle(QFrame::NoFrame);
     setWindowTitle(tr("Thread"));
     setAlternatingRowColors(act->isChecked());
     setRootIsDecorated(false);