Implemented <core-list> for the sidebar.

For #330. This breaks filters and search.
Started to implement <core-list> for plugin cards.

Much is broken: search, editor panels, sidebar links, CRCs, message and
Bash tag content display, and probably others too.
CSS tweaks to fix various elements' positioning.
Removed some icons from sidebar.

Removed priority and dummy plugin icons, they take up lots of space and
there must be better ways of displaying priority info.
Fixed sidebar navigation.
Fixed paper-item links not firing on edges.

The links were sitting in the middle of the items, with plenty of ripply
but non-performing space around them...
Reset list scrolling when changing game.
Split plugin editor into separate element.

Also fixed card flip effect.
More selective toolbar disable in edit mode.
Get rid of editor table animations.

Getting the <core-animated-pages> element to adjust to content height is
tricky and isn't really worth it.
Added data binding to <loot-plugin-card>.

Started to use it. Individual data members will need their own
observers, just like was the case in the Plugin object.
Added tag and message observers, fixed errors.
Fixed getting editable table content.
Added data binding to <loot-plugin-editor>.

Also fixed tooltip positions being wonky due to card transitions.
Plugin menu and editor states are now remembered.
Fixed error on core-list content change.
Re-implemented search as a filter.
Finished implementing content filter.

Now also checks versions, tags and messages. Also fixed the filter being
run twice.
Removed filter memory feature.

It never really worked. If need be, I'll re-implement it.
Started to refactor and flesh out filters.

New filter code goes in filters.js. Added messageless plugin filter, and
wrote message filters (but they're not implemented yet).