Added buttons to select gizmo operations from the toolbar

[adriengivry]

Description

• Added buttons to the toolbar to select a gizmo operation
• Buttons are updated to reflect the current state of the editor
• Added icons (3) for each operation (move, rotate, scale)

To-Do

• Fix the fact that changing operation mode only works because disabled buttons are clickable (bug Buttons can still be clicked when they are disabled #496). Once this bug is fixed, this functionality won't work.

Review Guidance

• Added editor actions to expose/centralize the feature (set/get current gizmo operation)
• Cleaned-up includes, and replaced the auto enable = []() { ... } lambda by an anonymous function, shared by both the editor mode and gizmo operation buttons to have a consistent disabled state

Screenshot(s)

OvEditor_lrplFvaZzY.mp4

Temporary design (showcasing functionality)

Final design