Improve target platform change UX

In app def, changing the target platform does not change package list right away. We want to keep that behavior and make it clearer from a user perspective.

• Target platform selection should be moved in a separate form, next to the current target platform display.
• Target platform selection should be hidden until a user clicks on a "change" button.
• User needs to select the new TP and save before we load an updated package list.