Improve multi-monitor support

[fabiospampinato]

There are a few problems when using this configuration in a multi-monitor setup, especially if those are arranged on the Y axis. I suspect some of them come from bugs in Phoenix itself. I think some of these problems won't happen if the monitors have all the same resolution and aren't arranged weirdly:

• Modals aren't centered properly.
• Centering windows may not work.
• Growing windows may not work.