Polymer Designer: initial version with PD in a <webview> in a dialog

CSS is half-right, the webview is fixed height and the guest doesn't resize.
Wrap Polymer Designer into a Dart polymer element
Move Polymer Designer code under src/
Cleanups
Fix ghost dialog
Style fixes; webview is still too short and doesn't resize with window
Add .gitignore
Works: generated code gets inserted in current file (designer is still in dialog)
Set code in PD to contents of current file: works with delay
setCode works with delay encapsulated in cde-polymer-element
Some renamings for clarity
Bug fixes
Some rearrangements and code fixes
Change minheight
Merge remote-tracking branch 'upstream/master' into polymer-designer
Get webview to autoresize

Puzzlingly, `autosize="on"` had exactly the opposite effect, despite the name and the documentation suggesting otherwise.
Change point of access to Polymer Designer to context menu item
Sync required Polymer version with the main app
Fix bug around reload() + renamings
works only if the selected tab is not changed