IDLE: Only display actual code context lines.

Instead of displaying a fixed number of lines, some blank, Code Context

now displays the variable number of actual context lines. When there

are no context lines, it shows a single blank line to indicate that the

feature is turned on.

The Code Context configuration option is changed from 'numlines'

(default 3) to 'maxlines' (default 15) to avoid possible interference

between user settings for the old and new versions of Code Context.

IDLE: Re-enable color configuration for Code Context.

The difference from before is that the settings are now on the

Highlights tab instead of the Extensions tab and only change one theme

at a time instead of all themes. The default for light themes is black

on light gray, as before. The default for the IDLE Dark theme is white

on dark gray, which better fits the dark theme.

When one starts IDLE from a console and loads a custom theme without

definitions for 'context', one will see a warning message on the console.

To stop the warning, go to Options => Configure IDLE => Highlights,

select the custom theme if not selected already, select 'Code Context',

and select foreground and background colors.