remove Grid struct, move cursor into own package file

cursor.go

@@ -0,0 +1,79 @@
+package main
+
+import (
+	ui "github.com/gizak/termui"
+)
+
+type GridCursor struct {
+	selectedID string // id of currently selected container
+	containers Containers
+	cSource    ContainerSource
+}
+
+func NewGridCursor() *GridCursor {
+	return &GridCursor{
+		cSource: NewDockerContainerSource(),
+	}
+}
+
+func (gc *GridCursor) Len() int             { return len(gc.containers) }
+func (gc *GridCursor) Selected() *Container { return gc.containers[gc.Idx()] }
+
+func (gc *GridCursor) RefreshContainers() {
+	gc.containers = gc.cSource.All().Filter()
+	if gc.selectedID == "" {
+		gc.Reset()
+	}
+}
+
+// Set an initial cursor position, if possible
+func (gc *GridCursor) Reset() {
+	if gc.Len() > 0 {
+		gc.selectedID = gc.containers[0].Id
+		gc.containers[0].Widgets.Name.Highlight()
+	}
+}
+
+// Return current cursor index
+func (gc *GridCursor) Idx() int {
+	for n, c := range gc.containers {
+		if c.Id == gc.selectedID {
+			return n
+		}
+	}
+	return 0
+}
+
+func (gc *GridCursor) Up() {
+	idx := gc.Idx()
+	// decrement if possible
+	if idx <= 0 {
+		return
+	}
+	active := gc.containers[idx]
+	next := gc.containers[idx-1]
+
+	active.Widgets.Name.UnHighlight()
+	gc.selectedID = next.Id
+	next.Widgets.Name.Highlight()
+
+	ui.Render(cGrid)
+}
+
+func (gc *GridCursor) Down() {
+	idx := gc.Idx()
+	// increment if possible
+	if idx >= (gc.Len() - 1) {
+		return
+	}
+	if idx >= maxRows()-1 {
+		return
+	}
+	active := gc.containers[idx]
+	next := gc.containers[idx+1]
+
+	active.Widgets.Name.UnHighlight()
+	gc.selectedID = next.Id
+	next.Widgets.Name.Highlight()
+	ui.Render(cGrid)
+}