DigitEntry: Draw up/down arrows instead of plus/minus

Using plus/minus is only useful for numeric columns. Using up/down is more
generic and looks better.
DigitEntry: Removed margin between columns

The button panel has no margins either and with the margin the controls
look a bit misplaced.
DigitEntry: Changed padding attribute to local variable

Create() is the only user
DigitEntry: Use Layout::GetTextPadding() instead of hardcoded value
DigitEntry: Moved constructor to source file
DigitEntry: Extracted CalculateLayout() method
Fonts: Renamed FontHeight variable to font_height
DigitEntry: Cache last_right value
DigitEntry: Removed trivial digit_width variable
DigitEntry: Increase the width of multi-character columns

e.g. the HOUR column or the UNIT column
DigitEntry: Recalculate layout in CreateTime() after column types are set
DigitEntry: Adjust scaling on touchscreen devices

Use the Layout::GetMaximumControlHeight() value for minimum control sizes.
DigitEntry: Use large font size for the values

The value is the thing that matters most in this dialog and should be
emphasized against the other controls.