The following procedure will configure X Windows so a long-touch (touch and hold) will simulate a right-click and display a context menu.

1. Install evdev and create a configuration directory.

   sudo apt install xserver-xorg-input-evdev
   sudo mkdir -p /etc/X11/xorg.conf.d/

2. Create the file /etc/X11/xorg.conf.d/90-right-click.conf with the following contents:

   Section "InputClass"
     Identifier "touchscreen right-click"
     Driver "evdev"
     MatchProduct "Microchip Technology Inc. AR1100 HID-MOUSE"
     Option "EmulateThirdButton" "1"
     Option "EmulateThirdButtonTimeout" "750"
     Option "EmulateThirdButtonMoveThreshold" "50"
   EndSection

3. Reboot

For more information about the options and their syntax please see the evdev manpage and the InputClass configuration options.