One component per file; GAME OVER offers CONTINUE

PRD G1 asks for one component per file. Eight files held two or
three; their eleven extra components move to files of their own,
helpers stay where both sides use them, and an archtest now fails
any file that holds two.

PRD 4.4 and the request say an error always offers CONTINUE.
ErrorState's button read RETRY; it now reads CONTINUE, under the
CONTINUE? countdown, on the crash screen and the lost page alike.

navigation/verticaltabs.go

@@ -0,0 +1,19 @@
+package navigation
+
+import (
+	"github.com/egoist/mygo/ui"
+)
+
+type VerticalTabsOptions struct {
+	Disabled bool
+}
+
+type VerticalTabsResult struct {
+	Element *ui.Element
+	Changed bool
+}
+
+// VerticalTabs are tabs down the panel's left edge.
+func VerticalTabs(c *ui.Context, text string, labels []string, selected *int, o VerticalTabsOptions, panel func()) VerticalTabsResult {
+	return VerticalTabsResult(tabs(c, text, labels, selected, true, TabsOptions(o), panel))
+}