* fixed lint errors in button.go
* fixed lint errors in checkbox.go
* Removed d2ui singleton, fixed nearly all lint errors
- Changed `UI` struct to `UIManager`, removed singleton
- UI element provider functions are now methods of the UI Manager
- Screens now use the UI manager to create UI elements
- game panels in d2player now use the UI Manager to create UI elements
- Only the UI manager knows about "widgets"; calls to `d2ui.AddWidget` in Screen instances have been removed
* changed ui element provider methods from `Create` to `New`
remove unused struct vars that were only stored in order to pass to the next screens
consolidate create game code to single method
export ScreenMode consts and SetScreenMode method to allow app to create the correct screens
Co-authored-by: carrelda <carrelda@git>
