Fix alignment of number pad buttons

[leduyquang753]

Fixes another instance of #127

Description of the changes

• Add ExtraColumns property to NumberPad control.
• Add an extra column to NumberPad control instances in standard, scientific and programmer button panels.

Additional information

The NumberPad control is placed as sort of a "subgrid" within the button panels' grids, on a subset of columns. This causes slight differences in how the column widths are computed in the outer button panel and in the number pad control (see CGrid::DistributeStarSpace) and makes the number pad buttons slightly misaligned with the surrounding buttons. Adding extra empty columns to the right of the NumberPad so that it reaches the right side of the button panel fixes this inconsistency.

How changes were validated

Manual testing.