Labels widget: tab and shift-tab support

[teosarca]

Follow-up of #1232.

In this task we shall implement the TAB and SHIFT-TAB support as described in #1232.

[metas-lc]

please also implement:

• "nice to have: pressing LEFT / RIGHT would be nice to navigate between labels and then pressing BACKSPACE i would delete the one i want."
• " when having the cursor in previous field (i.e. VAT ID), pressing tab shall take me to this labels field." - i have to press TAB multiple times to get there.

[siemiatj]

Related to #1617

[metas-lc]

IT

1. pressing LEFT / RIGHT => navigates between labels OK
2. pressing BACKSPACE => deletes the one i want OK
3. have the cursor in previous field (i.e. VAT ID), press tab => i get to labels field. OK
   .
4. if i am in label fields, select a label and press enter multiple times, that label is set again, each time NOK
   follow-up: Labels widget: pressing enter #1832