Allow lazy loading of DropDown widgets

[Arlo White]

Currently DropDown? widgets must specify a dropDown at creation.

Imagine you have a web application with a hundred DropDownButtons?. Having to instantiate all the TooltipDialogs? just about doubles your load time. This shouldn't be necessary when the user will likely only click a few of the DropDownButtons?.

There should be a way to create a DropDownButton? that creates its dropDown only after it's clicked.

Maybe _HasDropDown should have a createDropDown method that it calls if this.dropDown doesn't exist. Or something along those lines...

This seems like it should be easy enough. There are only two places this.dropDown is referenced:

DropDownButton? references this.dropDown in its startup, this code would need to adjust for the case where dropDown is lazy-loaded.

_HasDropDown references it in toggleDropDown, this is where you would call your createDropDown code if not already created.

[liucougar]

(In [22770]) fixes #11638: modify DropDownButton? to not assume dropDown is set upon creation of the button.

To lazy-load a dropdown menu, overwrite the default loadDropDown function to create the menu (and populate it) and attach it to this.dropDown

[bill]

In [26501]:

Backport [22770] (refs #11638 !strict) and also part of [22555], to 1.5, allowing lazy creation of DropDownButton drop down.

Backport [26469], [26500] (refs #13866 !strict) to 1.5, for lazy creation of Editor's foreColor/hiliteColor ColorPalette drop down.