Fix a number of focus-related issues

Fix a number of focus-related issues

The are a couple of issues with the way focusing is implemented (or
rather, neglected):
 - Focusing on a different client does not change the keyboard focus
 - The keyboard focus does not follow the pointer
 - The keyboard focus does not change when clicking on a window
 - The pointer does not follow the focus

This commit fixes the above issues by making the following changes:
 - Focus on a client when the pointer enters it
 - When focusing on a client, also move the keyboard focus
 - When focusing on a client, move the pointer over the client