Second try to fix drop down menu opening via ENTER/SPACE key.  Refactoring of _Has   - Switched keyboard handling to onkeyup so that drop down doesn't receive spurious keyup event.   - Separated keyboard and mouse handling into separate functions that don't call each other.   - Since _HasDropDown monitors mousedown and mouseup explicitly (and also keyup), it didn't seem necessary to connect to the onclick event at all.   That invalidates the _stopClickEvents flag; hopefully that doesn't break anything.   FilePickerTextBox still seems to work  and so does DropDownButton/ComboButton.    Refs #11040
Second try to fix drop down menu opening via ENTER/SPACE key.  Refactoring of _Has
  - Switched keyboard handling to onkeyup so that drop down doesn't receive spurious keyup event.
  - Separated keyboard and mouse handling into separate functions that don't call each other.
  - Since _HasDropDown monitors mousedown and mouseup explicitly (and also keyup), it didn't seem necessary to connect to the onclick event at all.   That invalidates the _stopClickEvents flag; hopefully that doesn't break anything.   FilePickerTextBox still seems to work  and so does DropDownButton/ComboButton.
  
Refs #11040

git-svn-id: 674a2d6b1e32e53f607eb824547723f32280967e@22001 560b804f-0ae3-0310-86f3-f6aa0a117693

harbalk/delite,Sam-Cummins/delite,Sam-Cummins/delite,kitsonk/delite,harbalk/delite,kitsonk/delite