Mouse/click improvements

[docelic]

After applying patch for mouse support, edit src/widget/button.cr as the prototype component where mouse behavior re. clicking is to be polished. Minimum improvements necessary:

1. Support hover over buttons
2. Support press/release events and make them affect current widget style (e.g. to give "button press" visual feedback)
3. If mouse moves out of button while clicked, releasing it will be no-op (like in GUIs)