Fix HiDPI issues for menus

Menus now use the size that users have set globally for their
applications. This is accomplished by removing the fixed font size
definition (in points) for `QMenu` in `style.css`.

In some places the menus had been set to hard coded font sizes. This
code is also removed and applies to the following menus:
* Combo box menus
* Track operation widget (gear icon on tracks)
* The MIDI port menu

data/themes/default/style.css

@@ -115,7 +115,6 @@ QSplashScreen QLabel {
 QMenu {
 	border-top: 2px solid #08993E;
 	background-color: #15191c;
-	font-size: 11px;
 }
 
 QMenu::separator {
@@ -133,15 +132,12 @@ QMenu::item {
 
 QMenu::item:selected {
 	color: #d1d8e4;
-	font-weight: normal;
 	background-color: #21272b;
 }
 
 QMenu::item:disabled {
 	color: #515459;
 	background-color: #262b30;
-	font-size: 12px;
-	font-weight: normal;
 	padding: 4px 32px 4px 20px;
 }
 

src/gui/menus/MidiPortMenu.cpp

@@ -34,7 +34,6 @@ MidiPortMenu::MidiPortMenu( MidiPort::Modes _mode ) :
 	ModelView( nullptr, this ),
 	m_mode( _mode )
 {
-	setFont( pointSize<9>( font() ) );
 	connect( this, SIGNAL(triggered(QAction*)),
 			this, SLOT(activatedPort(QAction*)));
 }

src/gui/tracks/TrackOperationsWidget.cpp

@@ -64,7 +64,6 @@ TrackOperationsWidget::TrackOperationsWidget( TrackView * parent ) :
 				"to begin a new drag'n'drop action." ).arg(UI_CTRL_KEY) );
 
 	auto toMenu = new QMenu(this);
-	toMenu->setFont( pointSize<9>( toMenu->font() ) );
 	connect( toMenu, SIGNAL(aboutToShow()), this, SLOT(updateMenu()));
 
 

src/gui/widgets/ComboBox.cpp

@@ -70,7 +70,6 @@ ComboBox::ComboBox( QWidget * _parent, const QString & _name ) :
 	}
 
 	setFont( pointSize<9>( font() ) );
-	m_menu.setFont( pointSize<8>( m_menu.font() ) );
 
 	connect( &m_menu, SIGNAL(triggered(QAction*)),
 				this, SLOT(setItem(QAction*)));