Don't treat SPACE as the start of a keyboard search.
Add KeyNavContainer robot test.
Fixes #17766.
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