Add truecolor support for Windows, if available (#2156)

- Update to latest tcell which has 24 bit Windows support
- light renderer under Windows defaults to Dark256, if possible
- Respect TCELL_TRUECOLOR
- Remove tcell 1.3 references

src/tui/light.go

@@ -3,7 +3,6 @@ package tui
 import (
 	"fmt"
 	"os"
-	"os/exec"
 	"regexp"
 	"strconv"
 	"strings"
@@ -126,17 +125,6 @@ func NewLightRenderer(theme *ColorTheme, forceBlack bool, mouse bool, tabstop in
 	return &r
 }
 
-func (r *LightRenderer) defaultTheme() *ColorTheme {
-	if strings.Contains(os.Getenv("TERM"), "256") {
-		return Dark256
-	}
-	colors, err := exec.Command("tput", "colors").Output()
-	if err == nil && atoi(strings.TrimSpace(string(colors)), 16) > 16 {
-		return Dark256
-	}
-	return Default16
-}
-
 func repeat(r rune, times int) string {
 	if times > 0 {
 		return strings.Repeat(string(r), times)