Fixed: Holding and releasing mouse button is not inserting autocomplete item

[engineering-this]

What is the purpose of this pull request?

Bug fix

Does your PR contain necessary tests?

All patches which change the editor code must include tests. You can always read more
on PR testing,
how to set the testing environment and
how to create tests
in the official CKEditor documentation.

This PR contains

• Unit tests
• Manual tests

What changes did you make?

I've registered autocomplete items in focusManager, so focusing them wont trigger editor blur event which was cause for hiding dropdown. Note tabindex is required for items to be focusable.

Closes #2107.

[engineering-this]

I'm thinking if we should treat this as feature instead of just fix.

[engineering-this]

I will rebase it once master is merged with major.

[Comandeer]

Using [tabindex] introduces several quirks.

The most visible of them is default styling for focusable elements, which appears after any mouse related action (e.g. right click and closing native context menu):

The second quirk is connected with the fact that focusing any element with mouse breaks keyboard behaviour. Tab no longer inserts selected item into the editor, but focus the next element in tab order (in the manual test case – sidebar close button), arrow keys scroll the page and Esc does nothing.

[Comandeer]

I'd rather add [tabindex] programatically, via element.$.tabIndex = -1 in addItem method. Requiring user to remember to add this attribute to itemTemplate seems unnecessary.

[engineering-this]

After reconsidering this issue I decided to drop changes. I didn't delete commits, so it can be reverted.

For proper usage of autocomplete editor should remain focused all the time, so we can continue typing. Hence it shouldn't register anything in focusManager. Instead solution is to
event.preventDefault(); on mousedown, which solves the problem.

Now fix contain less code, quirks described above don't happen, and also this change solves #2126.

[Comandeer]

LGTM!