ComboBox: first rough implementation (implements #112)

The PR contains an initial implementation of a ComboBox widget (see task #112), as a complementary widget of deliteful/Select.

Remarks:
* The short-term purpose of this PR is to allow getting a feeling about the current state of the widget. 
* For now, it only contains the widget code and theme, and a manual test file for the programmatic and markup use cases. 
* Should be added shortly: some minimal unit and functional automatic testing, user documentation (including jsfiddle), a small sample, and general code and API doc fixes and improvements.
* As agreed, keyboard navigation support will be added in the next development cycle (will create an issue to keep track of it).

Prerequisites:
* ibm-js/delite/pull/310 ("dialog" stuff). *Update*: now merged into delite's master.
* #334 (store custom element support) *Update*: now merged into deliteful's master.

Workarounds (for now):
* In the longer run, #341 (List styling issues) is also a prerequisite, but for now the PR adds by itself the pieces of this PR which it needs (such that we don't depend on #341 for now).
* ibm-js/delite/issues/317 is workarounded in this PR for now.
* A (dirty) workaround for an issue with bad pairing in List of the busy on/off state (apparently related with List.attachedCallback breaking the automatic chaining - see the code around https://github.com/ibm-js/deliteful/pull/344/files#diff-9d795a38517bb5a35c96adbcc5ec431bR426 - will enter an issue when I'll have time to produce a reduced sample reproducing).

Known issues:
* Various cosmetic and ergonomic glitches here and there...
* On mobile, popup sizing/position and scrolling issues as reported in ibm-js/delite/pull/310.


My reasoning was that "combobox" can be taken as a single word, even if of course it is composed from two words. To write it correctly in English I think it should be in two separate words (thus leading to "ComboBox" and "d-combo-box"). But there are quite a lot of occurrences of "combobox", while "starrating" or "scrollablecontainer" are hardly readable and we'd rather separate the words or use capitalization to make them readable. On the other side, it's true that the name of the class itself is capitalized "ComboBox", not "Combobox" - here I followed the capitalization we had in Dojo 1.x (both dijit and dojox/mobile) which can be seen in other GUI libs.

All in one, I did have some arguments in mind when I went for "d-combobox", but they are not really strong arguments.