Polymer UI menu: cleanups, refactorings, etc.
Added "change theme" and "change keys" to Polymer UI menu

Might be temporary, if can't make them work.
Fixed widgets and Polymer UI: menu selection is plumbed up to the UI now
Merge branch 'upstream/master'
Addressed comments
Merge branch 'upstream/master'
WIP: switching to MVC paradigm in Polymer UI

SparkPolymerUI receives a handle to SparkModel, which Spark and SparkPolymer implement. SparkPolymerUI uses the model to map various events in the UI to ActionManager calls, etc.

There is currently a race condition: Spark ctor wants to access various elements of the UI, and so the UI (SparkPolymerUI) gets created too early, before enteredView() fires for various components, leading to a crash in SparkSelector on a null _target.
Merge branch 'upstream/master'
Add assert
Fix unnoticed refactoring artifact
Merge branch 'upstream/master'
Temporarily patched the MVC scheme

Failed to use a full-on MVC with programmatic creation of SparkPolymerUI from SparkPolymer ctor: that's too late, because the super-ctor (Spark) queries the still-uncreated UI via querySelector, and thus crashes. After a switchover to Polymer and getting rid of querySelector's to do things, the full MVC will be possible to implement.
Fixed the menu in Polymer UI

Combination of 2 factors:
1) spark-selector couldn't return spark-menu-item in the fired event: that resulted in a quited program exit; the receiver never received the event. Switched spark-selector to use the item's value when attrvalue is defined, and defined it to "action-id" in spark-polymer-ui. This works.
2) The intermediate event receiver-resender in spark-menu-button for some reason crashed the app: the only menu item that worked was Open File, the rest caused even more of the DataCloneErrors (see below) and printed a stack trace.

Remaining errors:
1) There is a weird error printed to the console multiple times (on startup, on the menu open, on a menu item click): "Uncaught DataCloneError: An object could not be cloned." In most cases that doesn't throw an exception or print a stack trace. But in one case it does - see above. Will file a bug.
2) After opening the menu (even without clicking any items), the treeview misteriously breaks: a click on a file/folder collapses the toolbar. The rest of the treeview functions work and don't cause this effect. This is the same in the current main master too, so it's not this branch's doing.
Synchronized the menu contents with non-Polymer UI

Close and Delete got removed
Cleanup
Got the +/- buttons for themes and keys to work in Polymer menu

Remaining problems: styled ugly & closes the menu on click (can't cycle through themes/keys easily).
Small fix-up
Merge branch 'upstream/master'
Merge pull request #480 from ussuri/fix-polymer-ui-menu

Fix polymer UI menu