Move webview samples into their own directory

All samples that use webviews are moved to the a "webview-samples" directory.

NOTES:
- I have not looked into whether any samples that use webviews are "broader than just webview" (and therefore shouldn't live in a webview-specific folder)
- Two pending pull requests (see user-agent and new-window branches) contain changes to move their respective samples to the "webview-samples" directory