T-Mobile USA is updating its Sidekick cell phones, adding a high-end model and the first Motorola-built entry in the line of quirky gadgets with a screen that swivels to reveal a keyboard. The new luxury model, the Sidekick LX, has a screen with more than twice the resolution of the previous top-of-the-line model, the Sidekick 3. Criticism of the low screen resolution has dogged the line, which …