1.16 mouse input

The `x11`, `pm`, `windows`, `ggi`, and `wxt` terminals allow interaction with the current plot using the mouse. They also support the definition of hotkeys to activate pre-defined functions by hitting a single key while the mouse focus is in the active plot window. It is even possible to combine mouse input with `batch` command scripts, by invoking the command `pause mouse` and then using the mouse variables returned by mouse clicking as parameters for subsequent scripted actions. See bind and variables. See also the command `set mouse`.