Dialog: can't contain layout widgets

[bill]

Split off from #4169:

Since layout widgets can't render inside a hidden div (at least on some browsers), tabs etc. don't work correctly inside of a Dialog.

[bill]

(In [12161]) Fixes #5669: Dialog can't contain layout widgets. Modifies Dialog to initialize in a visibility:hidden state rather than display:none, in order for possible child layout widgets to initialize correctly. As Dialog is not a layout widget itself, note that this means that the child layout widgets must have a size specified (rather than specifying a size on the Dialog itself)

[bill]

(In [12163]) Put width on the dialog loaded from an href containing a <p>, so it doesn't become too wide and hit the edge of the viewport. All dialogs should have a specified width unless they are naturally narrow (because they contain a form or something like that).

Refs #5669.

[bill]

(In [12179]) If dialog is marked as display:none in markup, override it since we are making it visibility:hidden instead. Fixes dialog display in themeTester. Refs #5669.