Add footer

Options:
  --footer=STR             String to print as footer
  --footer-border[=STYLE]  Draw border around the footer section
                           [rounded|sharp|bold|block|thinblock|double|horizontal|vertical|
                            top|bottom|left|right|line|none] (default: line)
  --footer-label=LABEL     Label to print on the footer border
  --footer-label-pos=COL   Position of the footer label
                           [POSITIVE_INTEGER: columns from left|
                            NEGATIVE_INTEGER: columns from right][:bottom]
                           (default: 0 or center)

The default border type for footer is 'line', which draws a single
separator between the footer and the list. It changes its position
depending on `--layout`, so you don't have to manually switch between
'top' and 'bottom'

The 'line' style is now supported by other border types as well.
`--list-border` is the only exception.

src/tui/tcell.go

@@ -600,6 +600,8 @@ func (r *FullscreenRenderer) NewWindow(top int, left int, width int, height int,
 		normal = ColNormal
 	case WindowHeader:
 		normal = ColHeader
+	case WindowFooter:
+		normal = ColFooter
 	case WindowInput:
 		normal = ColInput
 	case WindowPreview:
@@ -865,6 +867,8 @@ func (w *TcellWindow) drawBorder(onlyHorizontal bool) {
 			style = ColListBorder.style()
 		case WindowHeader:
 			style = ColHeaderBorder.style()
+		case WindowFooter:
+			style = ColFooterBorder.style()
 		case WindowInput:
 			style = ColInputBorder.style()
 		case WindowPreview: