Allow mod authors to add big icons to admin menu

- add icon files to menu array if hook entry added file type and file exists (.gif and .png file types only)

Signed-off-by: -Underdog- github.underdog@gmail.com
Update Admin.template.php

- display custom icon if it was added to the array

Signed-off-by: -Underdog- github.underdog@gmail.com
Update admin.css

- add css for custom icon

Signed-off-by: -Underdog- github.underdog@gmail.com
end of files

removed empty line at end of files

Signed-off-by: -Underdog- github.underdog@gmail.com
Merge pull request #2374 from Underdog-01/patch-5

Allow mod authors to add big icons to admin menu