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/stackwindow.cpp

@@ -54,6 +54,7 @@ namespace Internal {
 StackWindow::StackWindow(DebuggerManager *manager, QWidget *parent)
     : QTreeView(parent), m_manager(manager), m_alwaysResizeColumnsToContents(false)
 {
+    setFrameStyle(QFrame::NoFrame);
     m_disassemblerAgent = new DisassemblerViewAgent(manager);
 
     QAction *act = theDebuggerAction(UseAlternatingRowColors);