frame-it: use Ubuntu Frame on desktop

*frame-it* makes it easy to use Ubuntu Frame on the desktop:

• frame-it.check checks Ubuntu Frame is installed plus options frame-it uses
• frame-it gnome-calendar will run gnome-calendar using Ubuntu Frame
• frame-it.x11 xeyes will run xeyes using Ubuntu Frame
• snap set frame-it shell-app=gnome-calendar sets the shell-app option
• frame-it.shell will run Ubuntu Frame using the shell-app option
• sudo frame-it.root-on-x11 will run Ubuntu Frame as root using X11
• frame-it.vnc user@host will connect to a remote host and run a VNC client

When shell-app is set a login shell is provided from the greeter: Logging into "frame-it" will run Ubuntu Frame using the shell-app option as a user session