PixelPaint: Specify histogram height in GML

The histogram is perfectly fine with being drawn at any size, but the
code currently fixes its height to 65. Once the histogram is in a
subclass and several GML things around it change, the fixed height
breaks, so we move the height specification to GML. Additionally, the
container is specified to shrink as much as possible, alleviating a
hard-coded UI size. The user can now change histogram height in GML,
which is a lot more obvious.

Userland/Applications/PixelPaint/HistogramWidget.cpp

@@ -16,11 +16,6 @@ REGISTER_WIDGET(PixelPaint, HistogramWidget);
 
 namespace PixelPaint {
 
-HistogramWidget::HistogramWidget()
-{
-    set_height(65);
-}
-
 HistogramWidget::~HistogramWidget()
 {
     if (m_image)

Userland/Applications/PixelPaint/HistogramWidget.h

@@ -24,7 +24,7 @@ public:
     void set_color_at_mouseposition(Color);
 
 private:
-    HistogramWidget();
+    HistogramWidget() = default;
 
     virtual void paint_event(GUI::PaintEvent&) override;
 

Userland/Applications/PixelPaint/PixelPaintWindow.gml

@@ -66,14 +66,14 @@
 
             @GUI::GroupBox {
                 title: "Histogram"
-                max_height: 90
+                preferred_height: "shrink"
                 layout: @GUI::VerticalBoxLayout {
                     margins: [6]
                 }
 
                 @PixelPaint::HistogramWidget {
                     name: "histogram_widget"
-                    max_height: 65
+                    min_height: 65
                 }
             }