Remove "GENERAL SETTINGS" tab

Remove the "GENERAL SETTINGS" tab from the instrument track and sample track window.

This removes clutter as well as dependencies to the non-scalable `TabWidget`.

Remove superfluous calls to `setSpacing` which have set the default value of 6.

Use the default content margins of (9, 9, 9, 9) instead of (8, 8, 8, 8).

src/gui/SampleTrackWindow.cpp

@@ -40,7 +40,6 @@
 #include "SampleTrackView.h"
 #include "Song.h"
 #include "SubWindow.h"
-#include "TabWidget.h"
 #include "TrackLabelButton.h"
 
 namespace lmms::gui
@@ -69,13 +68,9 @@ SampleTrackWindow::SampleTrackWindow(SampleTrackView * tv) :
 	vlayout->setContentsMargins(0, 0, 0, 0);
 	vlayout->setSpacing(0);
 
-	auto generalSettingsWidget = new TabWidget(tr("GENERAL SETTINGS"), this);
-
+	auto generalSettingsWidget = new QWidget(this);
 	auto generalSettingsLayout = new QVBoxLayout(generalSettingsWidget);
 
-	generalSettingsLayout->setContentsMargins(8, 18, 8, 8);
-	generalSettingsLayout->setSpacing(6);
-
 	auto nameWidget = new QWidget(generalSettingsWidget);
 	auto nameLayout = new QHBoxLayout(nameWidget);
 	nameLayout->setContentsMargins(0, 0, 0, 0);

src/gui/instrument/InstrumentTrackWindow.cpp

@@ -93,13 +93,9 @@ InstrumentTrackWindow::InstrumentTrackWindow( InstrumentTrackView * _itv ) :
 	vlayout->setContentsMargins(0, 0, 0, 0);
 	vlayout->setSpacing( 0 );
 
-	auto generalSettingsWidget = new TabWidget(tr("GENERAL SETTINGS"), this);
-
+	auto generalSettingsWidget = new QWidget(this);
 	auto generalSettingsLayout = new QVBoxLayout(generalSettingsWidget);
 
-	generalSettingsLayout->setContentsMargins( 8, 18, 8, 8 );
-	generalSettingsLayout->setSpacing( 6 );
-
 	auto nameAndChangeTrackWidget = new QWidget(generalSettingsWidget);
 	auto nameAndChangeTrackLayout = new QHBoxLayout(nameAndChangeTrackWidget);
 	nameAndChangeTrackLayout->setContentsMargins( 0, 0, 0, 0 );