Fix -1 offset in plugin tab

In the instrument plugin tab, there was an orange stripe for
TripleOscillator. This was because internally, TabWidget moves up the
widget by 1 (TabWidget.cpp, line 89).

The size of the whole window is:

```
widget->height() + m_tabbarHeight - 1
```

So this code adds an offset of "-1" to the necessary computations.

src/gui/widgets/TabWidget.cpp

@@ -321,7 +321,10 @@ QSize TabWidget::minimumSizeHint() const
 			maxWidth = std::max(maxWidth, it->w->width());
 			maxHeight = std::max(maxHeight, it->w->height());
 		}
-		return QSize(maxWidth + 4, maxHeight + m_tabbarHeight);
+		// "-1" :
+		// in "addTab", under "Position tab's window", the widget is
+		// moved up by 1 pixel
+		return QSize(maxWidth + 4, maxHeight + m_tabbarHeight - 1);
 	}
 	else {
 		return QWidget::minimumSizeHint();

src/tracks/InstrumentTrack.cpp

@@ -1439,7 +1439,10 @@ InstrumentTrackWindow::InstrumentTrackWindow( InstrumentTrackView * _itv ) :
 
 
 	m_tabWidget = new TabWidget( "", this, true, true );
-	m_tabWidget->setMinimumHeight( INSTRUMENT_HEIGHT + GRAPHIC_TAB_HEIGHT - 4 );
+	// "-1" :
+	// in "TabWidget::addTab", under "Position tab's window", the widget is
+	// moved up by 1 pixel
+	m_tabWidget->setMinimumHeight( INSTRUMENT_HEIGHT + GRAPHIC_TAB_HEIGHT - 4 - 1 );
 
 
 	// create tab-widgets
@@ -1857,7 +1860,10 @@ void InstrumentTrackWindow::viewPrevInstrument()
 
 void InstrumentTrackWindow::adjustTabSize(QWidget *w)
 {
-	w->setMinimumSize(INSTRUMENT_WIDTH - 4, INSTRUMENT_HEIGHT - 4);
+	// "-1" :
+	// in "TabWidget::addTab", under "Position tab's window", the widget is
+	// moved up by 1 pixel
+	w->setMinimumSize(INSTRUMENT_WIDTH - 4, INSTRUMENT_HEIGHT - 4 - 1);
 }
 
 #include "InstrumentTrack.moc"