the uiManager now handles every element of the ui, so we don't need to
render elements manually in game_controls. Now we can also use
widget_groups to simplify handling the opening/closing of the panel.
this allows us to groups screens together, such that they can be
de-/activated by de-/activating the group instead of every ui element by
hand.
Before we were deactivating buttons and stopped rendering to deactivate ui
elements. This tied the renderer to these elements.
* 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`
