DragValue keyboard operation is broken

[mwcampbell]

I observe these three related issues which I want to fix in one PR:

1. DragValue can't be incremented and decremented with the keyboard. In theory the code does this when the button form of the widget has focus, but the button never has focus.
2. When focus is moved to DragValue by tabbing, the widget is rendered in button mode for one frame, because egui doesn't know that the widget is interested in receiving focus until it is rendered as a button. This instability isn't good for screen readers.
3. If the DragValue widget is focused, then the user moves to an associated slider with Shift+Tab, adjusts the slider, then returns to the DragValue, the previous edit string is still there, and it overrides the change the user made to the slider.