Further UI fixups

More work to unify the tab view with the overall
UI look-and-feel, allowing you to remove and add
tabs via mouse over rather than the close tab
icon. Also added a number of cosmetic adjustments.

Fixed up PerformanceManager and ScatterPlot to
pick up on the chart fonts and tick size etc.

Removed some erroneous static members in the
ScatterPlot that caused a SEGV when adding multiple
charts (or cancelling an add then adding again).

src/RideMetadata.cpp

@@ -52,6 +52,10 @@ RideMetadata::RideMetadata(MainWindow *parent) : QWidget(parent), main(parent)
     tabs->setPalette(palette);
     tabs->setAutoFillBackground(false);
 
+    tabs->setStyleSheet("QTabBar { alignment: center; }"
+                          "QTabWidget::pane { top: 0px; }"
+                       );
+
     mainLayout->addWidget(tabs);
 
     // read in metadata.xml and setup the tabs etc