Unable to select custom filter conditions using keyboard

[szpak]

Describe the bug
While it is possible to choose AND, OR, NOT in the "Task Filter Manager" dialog, "Add condition" selector does not react on keyboard key strokes.

To reproduce
Steps to reproduce the behavior:

1. Go to 'Task Filter Manager'
2. Click on 'Add'
3. Press tab a few times to focus on the "+" button/icon
4. Press "Enter"
5. Try to select using a keyboard any condition in the "Add condition" selector.

Expected behavior
It should be possible to use ↓/↑/ENTER navigate in that selector to choose requested conditions.

Version & system information
App 0.8.3 (f3264fe).

Additional context
CTRL-ENTER should close the "Task Filter Manager" dialog (and many other.