Allow docking Line widgets

[docelic]

There exists 'border docking' functionality in Crysterm where overlapping/crossing borders are replaced with a more suitable character that joins them more seamlessly.

An example can be seen in examples/chat.cr on the bottom-right. (Can try the example with dock_borders: false to see the difference).

Similar functionality would be required for Line widgets as well. See test/widget-line.cr.

Docking functionality should be extracted into a reusable component, and then use it separately for borders and line widgets.