UI: Disable properties button in source toolbar

[cg2121]

Description

If a scene or group is selected, disable the properties button in the source toolbar. This also does a refactor of the function to enable/disable the toolbar buttons.

Motivation and Context

Makes the source toolbar consistent with the source context bar.

How Has This Been Tested?

Selected/unselected a nested scene in the sources list to make sure the toolbar updated correctly.

Types of changes

• Tweak (non-breaking change to improve existing functionality)
• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.

[COOLIGUAY]

Should it be the same for groups? Groups also have no properties.

[cg2121]

Updated to also include groups.