Fix menu positioning when opened via keyboard.  Fixes #17769.

Changes include:
  - make keydown handler process context-menu key in addition to shift-F10
  - (for keyboard) open menu around focused node rather than node matching
    this.targetNodeIds / this.selector
  - different approach to avoid repeat contextmenu key event;
    1ms delay in _scheduleOpen() was too short; there's about 100ms between
    the keydown and contextmenu events on IE8
remove unneeded CSS that flummoxes IE, fixes #17672
fix tooltips on SVG nodes on FF, fixes #16310
fixes #17751 use native placeholder support for form inputs
reuse background iframes between (non-currently shown) popups, fixes #17774
Added unit test assertion for #17746
fix dijit typos, refs #17778
Prevent container scrolling in IE on long tree labels

Fixes #17765.
Revert "fixes #17751 use native placeholder support for form inputs"

This reverts commit 63486c0569d7ba60e351cf01f36dd8f007ed3b48.

This commit caused backwards-incompatible changes to the toolkit
that will break existing users, and introduces a feature that
is not cross-browser compatible (HTML5 placeholder styling is
not consistent) in order to fix an unrelated defect.
fix #16482 Regression: _WidgetBase#placeAt() can't be used on a
DocumentFragment

* added tests for placement using document fragment
* updated WidgetBase.placeAt to allow a document fragment
Limit background iframe to IE; it's no longer needed on other browsers.
Also switch test from java applet to PDF, since the java applet wasn't loading correctly.
Fixes #17811, refs #11822.