Better support for Windows terminals

* Default border style on Windows is changed to `sharp` because some
  Windows terminals are not capable of displaying `rounded` border
  characters correctly.
* If your terminal emulator renders each box-drawing character with
  2 columns, set `RUNEWIDTH_EASTASIAN` environment variable to `1`.

src/tui/tui.go

@@ -410,6 +410,7 @@ type Renderer interface {
 	RefreshWindows(windows []Window)
 	Refresh()
 	Close()
+	NeedScrollbarRedraw() bool
 
 	GetChar() Event