Add configurable window border for terminal window

[o-sdn-o]

Changes:

• Revise title/footer rendering in GUI mode (fix shadows and overbright). Graphical front-end (GUI-TUI Bridge) #571
• Add a side border option config/term/layout/border (=0 by default). [feature request] configurable window border #584:

<config>
    <term>
        <layout>
            <oversize=0 opacity=0xC0/>  <!-- Scrollback horizontal (left and right) oversize. (for convenient horizontal scrolling). -->
            <border=1/>                 <!-- Viewport's left and right border. -->
        </layout>
    </term>
</config>

• config/term/fields section renamed to config/term/layout;
• config/term/fields/size option renamed to the config/term/layout/oversize section;
• config/term/fields/lucent option renamed to config/term/layout/oversize/opacity;