Disabled "Collapse" button in "Flat" view

[AdwaitSalankar]

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Description

Used setEnabled to disable bCollapseAll in flat mode and enabled it in the other modes.

Related Issue

Fixes #1784

How Has This Been Tested?

Wrote tests for the behaviour in test_extract.py and test_diff.py

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have read the CONTRIBUTING guide.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.

[AdwaitSalankar]

@real-yfprojects Can you review this PR?

[real-yfprojects]

Sorry for the delay @AdwaitSalankar. Glad this fix is merged now!