Improve Bake operator activation

[keianhzo]

Bake operators are not currently properly enabled/disabled based on the target objects state.

STRs:

• Add a linked reflection probe to the scene
• Go to the scene options
  Actual Result: The bake buttons are enabled
  Expected Result: The bake buttons shouldn't be enabled as the baking is not possible

This PR splits the current Bake operator in three different operators that have a different poll implementations.

This PRs splits the BakeOperator implementation in a base implementation and a concrete implementation for each bake type so we can poll properly. Although functional I'm not really happy with this implementation, I think there should be an easier way of making poll conditional but I couldn't find a better way based on my attempts below but I'm more than happy to know what should be (if any) the best way of doing this.

Other approaches considered:
The first intent was to store the bake mode in the context via context_pointer_set. This require a pointer type so storing a string/enum/int is not allowed:

            row = col.row()
            row.context_pointer_set('bake_op', 'ALL')
            bake_msg = "Baking..." if probe_baking and bake_mode == 'ALL' else "Bake All"
            bake_op = row.operator(
                "render.hubs_render_reflection_probe",
                text=bake_msg
            )
            bake_op.bake_mode = 'ALL'

Ideally we could store the operator properties reference but that would require the context_pointer_set to be called after the operator is created but in that case the context reference has already being bound and the context won't reflect the later context update:

            row = col.row()
            bake_msg = "Baking..." if probe_baking and bake_mode == 'ALL' else "Bake All"
            bake_op = row.operator(
                "render.hubs_render_reflection_probe",
                text=bake_msg
            )
            row.context_pointer_set('bake_op', bake_op)
            bake_op.bake_mode = 'ALL'

Another alternative would have been to add a property to the UILayout type to store the bake mode but doesn't seem reasonable to pollute the whole type with a narrow scoped property.:

            row = col.row()
            row.bake_mode = 'ALL'
            row.context_pointer_set('bake_data', row)
            bake_msg = "Baking..." if probe_baking and bake_mode == 'ALL' else "Bake All"
            bake_op = row.operator(
                "render.hubs_render_reflection_probe",
                text=bake_msg
            )