Feature: A mouse key can be assigned to open AppGrid

[Eraldo-Bako]

Added an option that lets users assign a mouse key that will open the GNOME App Grid menu by clicking the logo icon. It is disabled by default, and does not conflict the already existing 'open Activities' option.

Key Details:

• Heavily based on the existing logic used for "Icon Click Type to open Activities";
• Adds a "Disabled" state option (0 === Disabled);
• If a specific mouse button is already assigned to the Activities option and the user selects it for the App Grid, the Activities option automatically switches to Disabled (and vice versa) to prevent conflicts;
• The implementation complements the already existing structure, so active maintainers can easily add to/modify if needed.

Preview: