Custom listview layouts

Custom listview layouts

This changeset adds support for user-configured layouts in ListViews
and ExpandableListViews.

There is a new "Preview List Content" pullright menu when you right
click on a ListView in the layout canvas. The menu contains links to
configure
- the layout to use for list view items
- the layout to use as a header
- the layout to use as a footer
- as well as a number of pre-configured framework layouts for common
  arrangements like 2-item list, 1-item list, checked list, etc.

The selected layout is persisted as an inline comment in the layout
XML. (Current bug: When you drag elements around in the layout,
comment nodes are not preserved, meaning you will lose your list view
render type as well.)

This changeset also adds "Clear" to the resource chooser, which makes
it possible to clear a previously set value (until this, choosing
nothing would simply abort the selection.) This makes it possible to
remove a chosen header/footer.

Change-Id: I611b00f627c5773f3fe2443a6e16c29388687b36