FIX Skip autofocus on TreeDropdownField

To avoid (visually) unsetting the selected value if the drop down
is the first form field on the page (see LeftAndMain.EditForm.js)
FIX Only allow one concurrent ajax in TreeDropdownField

Prevents situations where a previous action might finish out of line
and override later choices, such as first expanding the (full) tree panel,
then limiting the tree through searching.
Reset TreeDropdownField search field after defocus

Was unsetting original selection even if no new one was made.