Added support for icons in semantic theme

[MacRusher]

Adds ability to use icons in TextField component.

Example usage:

<TextField name="username" icon="user" />
<TextField name="query" iconLeft="search" />

There could be only one icon, and icon take precedence before iconLeft.

I could also add support for pointer events if you think it would useful:

<TextField name="username" icon="add user" onIconClick={() => {}} />

One problem I'm aware of is that adding ui icon input classes change the input width.
This could be solved by adding fluid class either by TextField itself (IMHO bad idea) or end user:

<TextField name="username" className="fluid" icon="user" />

[MacRusher]

Few changes.

Per @radekmie recommendation I've introduced new section for the input which may or may not contain icon classes. So the problem with different input sizes is gone now.

Instead of onIconClick etc. I've also introduced iconProps that are passed directly to <i/> element.
Example usage:

<TextField name="username" icon="add user link" iconProps={{ onClick: () => {} }} />

[radekmie]

Great! Would you like to do the same for DateField and NumField also?

[MacRusher]

Sure, why not. I'll take care of this tomorrow.

[MacRusher]

I've added icon support for DateField and NumField

[radekmie]

Great!