This patch adds a 'MoveAmount' enum that permits movements by integer
values or to abstract extremes which loosely translate to "top" for up
direction, "bottom" for down direction, etc. The goal behind the effort
is to add support for buttons like "Home" and "End".
The new enum has a 'Default' impl that yields a value of integer
movement by 1 field. This replaces the previous 'Option' wrap, that
in fact served the same purpose, with the value of '1' scattered
around different places in code.
Along with the enum, new commands have been defined to make use of it:
- "move top"
- "move bottom"
- "move leftmost"
- "move rightmost"
