fix(UI): Preload viewwait's throbber image (refs #4978)

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 #1273 from eguaj/3.3-A4978

fix(UI): Preload viewwait's throbber image (closes #4978)