Nav tab renamed to Plugins

Also moved search box tooltip as it was being obscured by plugin cards.
Polymerised about dialog.
Polymerised first-run and progress dialogs.

I decided not to use paper-progress as it's less simple to animate.
Started to polymerise settings dialog.

However, the editable table still needs polymerisation, and there are
issues with getting and setting values, thanks to API differences from
standard elements (which is the only downside to Polymer I've come
across).
Fixed unregistered element usage.

Also moved my includes after Polymer init, as they'll probably be
polymerised shortly.
Added <loot-dialog> custom element.

This is a <paper-dialog> with a <dialog>-like API.
Fixed unclosed elements in <loot-dialog>.
Added <loot-dropdown-menu> custom element.

It's a <paper-dropdown-menu> with a value attribute.