[TYPO3-core] Tools>UserAdmin made skinnable

[Martin Kutschker]

Hi!

I've changed the module to use the icon API. The changes are attached to 
the file.

I updated the old SU-icons to have a small gap to the right and created new 
ones for the t3skin.

I have modified the tooltips for the SU icons. Both have now an add-on. 
[change-to mode] and [switch-back mode] respectively. The old [SU-mode] 
said nothing new as *both* icons had "SU" on it.

Masi