Dropdown component improvements

[mbezhanov]

This PR proposes 2 improvements to the Dropdown component:

1. The ability to specify fluid Dropdowns
2. Respond to "click" events instead of "focus" events, when selecting Dropdown items, and stop event propagation, in order to avoid conflicts with Modal component (this screenshot explains the reason behind this - if the Dropdown options overlap with the Modal dimmer, when trying to select the option, the Modal closes)

Used both in a project of mine. If you think it's appropriate, please consider accepting this PR. If any additional work is necessary on my end - let me know.

[almino]

First of all, sorry for not responding in a while. I had an accident and broke my elbow and femur. My left arm will be limited for a long time.

1. Sorry, I forgot that.

2. The click event has problem when I'm using Search Selection Dropdown. We have to work around this.

[almino]

I'm working on Multi Select Dropdown. When it make the master branch, it will include fluid attribute.

I'll try the @click.stop event.