Fixes to Input and CheckBox when updating styles

[drpepper]

Hello,

As part of a project I'm working on, I ran into two issues with PixiUI that I was able to resolve. I hope this can help others as well.

1. In my project, the Input fields can lose focus when in the "mobile simulator mode" of Chrome. This doesn't happen in the Storybook demo, I'm not sure why. I fixed it by adding a handler to the "blur" event on the input field.
2. Changing the background (setting the bg attribute) of an Input or CheckBox lead to strange results. For Input boxes, I was unable to interact with them, and white boxes appeared behind them. My checkbox sprites simply disappeared. I fixed this for the Input box by removing the old background and input mask before adding them back in, in the correct place. For the checkbox, I had to remove the active attribute so that the setter worked correctly.
3. I also added in preventDefault() to the keyboard input events, which should hopefully correct Text input may need to prevent default/bubble #45

Love the work you're doing, keep it up!

[CyberDex]

Thank you, @drpepper!!!