Fix issues in tcell renderer and Windows build

- Fix display of CJK wide characters
- Fix horizontal offset of header lines
- Add support for keys with ALT modifier, shift-tab, page-up and down
- Fix util.ExecCommand to properly parse command-line arguments
- Fix redraw on resize
- Implement Pause/Resume for execute action
- Remove runtime check of GOOS
- Change exit status to 2 when tcell failed to start
- TBD: Travis CI build for tcell renderer
    - Pending. tcell cannot reliably ingest keys from tmux send-keys

src/util/util_unix.go

@@ -15,3 +15,8 @@ func ExecCommand(command string) *exec.Cmd {
 	}
 	return exec.Command(shell, "-c", command)
 }
+
+// IsWindows returns true on Windows
+func IsWindows() bool {
+	return false
+}