After upgrading to the OS X 10.9 Mavericks Developer Preview, we proceeded to install one of our favorite everyday OS X apps: SizeUp. This neat little app, which we’ve used for years, allows you to quickly snap and position windows on your screen using keyboard shortcuts. SizeUp uses OS X’s GUI scripting control features to work its magic, and it requires you to “enable access for assistive devices” in System Preferences each time you install it on a new computer.…