Update SplitView API

API changes include:
- Removed togglePane method.
- Added hiddenDisplayMode property. Values:
  - none: When hidden, the pane is invisible.
  - inline: When hidden, the pane takes up space causing the SplitView's
    content to shift.

Now that there's an explicit mode for making the hidden pane invisible
(rather than an implicit mode by styling the hidden pane's width/height to
0), we can provide a default height/width for the hidden pane. Styling
changes:
  - Placement=top/bottom
    - Hidden height: 0px to 24px (matches minimal AppBar)
    - Shown height: 320px to 60px (matches shown AppBar with labels)
  - Placement=left/right
    - Hidden width: 0px to 48px
    - Shown height: 320px (no change)
SplitView: Avoid unnecessary measurement

When hiddenDisplayMode=none, we know that the hidden pane's thickness is 0
so we don't need to measure it.