sheet: support hyperlink-like capability in cells

[liquidaty]

Use case example: pivot table (see #293) drill-down.

After running and viewing a pivot table, each cell of the first column of the table should be visually different (e.g. blue+underline) to indicate that it has a "hyperlink"-like capability. When the user navigates to any such cell and presses Return, it should initiate a callback that "drills-down" into that cell and shows a new buffer with only the rows of data that correspond to that single row of the pivot table.

Implementation can be:

1. Extend key event handler registration to offer buffer-specific event handler
2. Add API function to specify a list of screen_buffer cells and their related format; store this as a 2D vector element stored as an additional property of the screen buffer struct

[liquidaty]

WIP: #296

[liquidaty]

See #302

[liquidaty]

Demonstrated in mysheet_extension.c