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/breakwindow.cpp

@@ -83,6 +83,7 @@ BreakWindow::BreakWindow(Debugger::DebuggerManager *manager)
   : m_manager(manager), m_alwaysResizeColumnsToContents(false)
 {
     QAction *act = theDebuggerAction(UseAlternatingRowColors);
+    setFrameStyle(QFrame::NoFrame);
     setWindowTitle(tr("Breakpoints"));
     setWindowIcon(QIcon(":/debugger/images/debugger_breakpoints.png"));
     setAlternatingRowColors(act->isChecked());