Prevent mouse click on input text from clearing text

[zachary1006]

As noted in #646, clicking inside a power-select-multiple input field after typing text clears the text. This proposed fix stops the propagation of the mousedown event if there is text in the input field.

To reproduce:
This issue can be seen on the Ember Power Select multiple-selection page (https://ember-power-select.com/docs/multiple-selection). To reproduce, type some text into the input field (do not press "Enter"). Then click your mouse in the input field, as though you were adjusting the cursor on a Word document. The text vanishes and the dropdown is closed.

Typing in the input

Text disappears on click

Expected behavior:
The cursor can be repositioned by clicking the mouse.

Cursor can be repositioned by clicking in the input

[dwickern]

@cibernox Could you take a look at this PR?