Add missing "Up" and "Left" Buttons to Fixture Group Editor

[yestalgia]

Overview

This pull request enhances the Fixture Group Editor in QLC+ by adding "Up" and "Left" buttons. These buttons allow users to add fixtures to a group starting from the selected cell in the reverse directions (upwards and leftwards) of the existing "Right" and "Down" buttons.

Changes Made

Code Enhancements:

1. Added new slots: slotUpClicked() and slotLeftClicked().
2. Extended addFixtureHeads() to handle Qt::UpArrow and Qt::LeftArrow.

UI

4. Added "Left" and "Up" buttons to the layout in fixturegroupeditor.ui.
5. Configured the buttons with an appropriate tooltip and icon (:/back.png, :/up.png).
6. Connected the buttons to slots in the constructor of FixtureGroupEditor.

Testing:

I have verified the functionality of "Up" and "Left" buttons across various scenarios, ensuring correct behaviour and bounds handling.

[yestalgia]

We may need to update translations for the hover text.

[coveralls]

coverage: 31.49% (-0.01%) from 31.5%
when pulling bc86fe1 on yestalgia:add/fixturegroupeditor-up-left-buttons
into 3547c9e on mcallegari:master.