PixelPaint: Add luminosity masking for editing masks

This adds a function where editing masks can be refined by selecting
a luminosity range that is applied to the content image and mapped to
the editing mask. This function allows the editing of image regions
that match only certain luminosity values.

Userland/Applications/PixelPaint/LuminosityMasking.gml

@@ -0,0 +1,65 @@
+@GUI::Frame {
+    fill_with_background_color: true
+    layout: @GUI::VerticalBoxLayout {
+        margins: [4]
+    }
+
+    @GUI::Widget {
+        layout: @GUI::VerticalBoxLayout {}
+
+        @GUI::Label {
+            name: "hint_label"
+            enabled: true
+            fixed_height: 20
+            visible: true
+            text: "Restrict mask to luminosity values:"
+            text_alignment: "CenterLeft"
+        }
+
+        @GUI::HorizontalRangeSlider {
+            name: "full_masking"
+            max: 255
+            min: 0
+            lower_range: 25
+            upper_range: 230
+            page_step: 10
+        }
+
+        @GUI::Widget {
+            name: "range_illustration"
+        }
+
+        @GUI::HorizontalRangeSlider {
+            name: "edge_masking"
+            max: 255
+            min: 0
+            lower_range: 0
+            upper_range: 255
+            page_step: 10
+        }
+
+        @GUI::CheckBox {
+            name: "mask_visibility"
+            text: "Show layer mask"
+        }
+
+        @GUI::HorizontalSeparator {}
+    }
+
+    @GUI::Widget {
+        layout: @GUI::HorizontalBoxLayout {}
+        fixed_height: 22
+
+        @GUI::Layout::Spacer {}
+
+        @GUI::DialogButton {
+            name: "apply_button"
+            text: "OK"
+        }
+
+        @GUI::DialogButton {
+            name: "cancel_button"
+            text: "Cancel"
+        }
+    }
+}