cr_elements demo_element cleanup
This CL:
* Adds <content> to cr_expand_button and cr_toggle_button for consistency with other buttons which allow <content>.
* Adds additional entries and content so that spacing and sizing can be better evaluated.
* Moves the networking elements to a separate expandable section and adds a cr-network-list example.
* Includes some minor formatting / 1.0 fixes.
BUG=none
Review URL: https://codereview.chromium.org/
1214483003
Cr-Commit-Position: refs/heads/master@{#337464}