Full screen is derived state, not a mode of its own. Live preview is
full screen exactly when the editor is collapsed and the sidebar is
away, so it also turns on when you enter design mode and then collapse
the sidebar by hand, which previously left the expand icon out of sync
with what was on screen.

Exiting undoes only what entering did. Entering from the code editor
turns design mode on and hides the sidebar, so exiting restores both.
Entering from design mode only hides the sidebar, so exiting brings the
sidebar back and stays in design mode instead of jumping to the editor.

The sidebar toggle keeps working as it always did. Showing the sidebar
from full screen drops you into design mode on its own, so it needed no
special casing.

Two other bits:
- Resizer persists visible:false when the sidebar hides, but full screen
  is not a persisted mode, so quitting from it would reopen the app with
  no sidebar and no clue why. The stored flag is put back after hiding.
- The sidebar drag handle is hidden in full screen. It sits invisible
  over the left edge of the preview and would otherwise steal clicks.

The expand icon in the live preview toolbar was just a second copy of
the design mode toggle, which is not what the icon says it does. It now
expands live preview to full screen, and design mode stays its own
thing on the control bar.

Renamed the button to fullScreenLivePreviewButton since the old id said
design mode, and pointed its icon and tooltip at the full screen change
event instead of the design mode one.

The mode dropdown chevron keeps hiding in design mode as before, now
driven directly by the design mode event.