Make arrow "stepper" buttons themeable and use SVG assets (#8123)

* SVG-ify arrow buttons

These buttons are used in the instrument window, Vestige, and VST
effects. Separate versions of the arrow icons are used for the classic
theme.

* Fix some unrelated SVG formatting and metadata

* Revert accidental change to classic border radius

* Add some XML stuff back to appease Github

LMMS renders these SVGs just fine, but apparently the removal of the
XML declaration completely breaks Github's ability to render the image,
so I am adding these back for the sake of those who want to actually
look at the diff on the website lmao

* Attempt to fix Github SVG previews again (`xmlns`)

* Fix crossover eq band mute button icon size

You may ask, "what does this have to do with the arrow buttons?" and you
would be right to assume this is unrelated. However, I'm already
touching the relevant lines of the stylesheet so I may as well sneak it
in there.

* Add missing metadata to fader_knob.svg

And fix mixed indentation in headphones.svg

* Add missing `xmlns` to fader_knob.svg

GRADIENTS!!!!!!!!!!!!

* Fix classic theme arrow buttons

The originals were right angle chevrons

* Remove unused getters

include/LeftRightNav.h

@@ -25,7 +25,7 @@
 #ifndef LMMS_GUI_LEFT_RIGHT_NAV_H
 #define LMMS_GUI_LEFT_RIGHT_NAV_H
 
-#include "PixmapButton.h"
+#include <QPushButton>
 
 
 namespace lmms::gui
@@ -36,16 +36,14 @@ class LeftRightNav : public QWidget
 	Q_OBJECT
 public:
 	LeftRightNav(QWidget *parent=nullptr);
-	PixmapButton* getLeftBtn();
-	PixmapButton* getRightBtn();
 	void setShortcuts(const QKeySequence &leftShortcut=Qt::Key_Minus, const QKeySequence &rightShortcut=Qt::Key_Plus);
 signals:
 	void onNavLeft();
 	void onNavRight();
 private:
 	QHBoxLayout m_layout;
-	PixmapButton m_leftBtn;
-	PixmapButton m_rightBtn;
+	QPushButton m_leftBtn;
+	QPushButton m_rightBtn;
 };