Multiple fixes to mixins

• Fixed columns mixin issue for using px instead of % values since it
breaks if it uses a percentage value (line 27)
• Updated drop-down mixin to use variable values (line 196)
• Updated button mixin for complex version (line 265)
• Updated icon mixin to include a variant value for lighter icons
(lines 286-460)
• Update animate mixin to include a fillmode variable (line 552)