fix(UI): Preload viewwait's throbber image (refs #4933)

Call viewwait(false), on document's onload, to pre-create a hidden
throbber element to allow the throbber's image to be pre-loaded before
the first use.
Merge pull request #1245 from eguaj/3.2-A4933

fix(UI): Preload viewwait's throbber image (closes #4933)