add (non-focused) keyboard scrolling to grid view pages

ui/browsing/browsingpane.go

@@ -30,10 +30,17 @@ type Searchable interface {
 	SearchWidget() fyne.Focusable
 }
 
+// Pages with selection should implement this interface to receive Ctrl+A events
 type CanSelectAll interface {
 	SelectAll()
 }
 
+// Pages that have one main scrollable view should implement this interface
+// to receive callbacks from window-level keyboard scrolling (up/down)
+type Scrollable interface {
+	Scroll(amount float32)
+}
+
 type CanShowNowPlaying interface {
 	OnSongChange(song, lastScrobbledIfAny *mediaprovider.Track)
 }
@@ -169,6 +176,18 @@ func (b *BrowsingPane) SelectAll() {
 	}
 }
 
+func (b *BrowsingPane) ScrollUp() {
+	if s, ok := b.curPage.(Scrollable); ok {
+		s.Scroll(-75)
+	}
+}
+
+func (b *BrowsingPane) ScrollDown() {
+	if s, ok := b.curPage.(Scrollable); ok {
+		s.Scroll(75)
+	}
+}
+
 func (b *BrowsingPane) doSetPage(p Page) bool {
 	if b.curPage != nil && b.curPage.Route() == p.Route() {
 		return false