debugui: make Number(F) close to more common widgets

[hajimehoshi]

• +/- buttons should be put in the vertical order
• Opening text field by Shift+Click is not intuitive. This should be editable by clicking directly.
• Keeping +/- pressed should increase/drecrease values repeatedly.
• Pressing up/down arrow keys should increase/drecrease values.

/CC @wasedaigo

[hajimehoshi]

Pressing up/down arrow keys should increase/drecrease values.

This might not be usual, and requires some refactoring. Let's not do it and revisit this later.

[hajimehoshi]

Pressing up/down key seems common in HTML: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/number