Added 'match_on_display_string' boolean to MenuSection.

Is set to the true by default.

Consider 'source code - edit (sce)'.

When match_on_display_string is true the user may type any of ...
    sou
    sour
    source
    sce
... to execute the command.

When match_on_display_string is false the user may type only ...
    sce
... to execute the command.

This is useful when, for example, 'source' may be used elsewhere in the system.
Then setting macth_on_display_string=False can eliminate command aliasing.