Rename some Dropdown-related properties

[jeffdaley]

Changes the name of some selected properties to isSelected to differentiate them from other selected properties.

In the context of DropdownLists, we use selected in two ways:

1. "Among a list, which item(s) [are] selected?"
2. "Is this item selected?"

It's hard to remember which one is which, so I'm renaming the latter cases.

Here's a quick visualization of the changes:

[jfreda]

Makes sense!