[project_properties] checkUseTabs with QComboBox,PEP8 Fix

- Replace old QCheckBox on checkUseTabs with new QComboBox just like on Preferences.
- Fix method calling QFileDialog incorrectly on _load_python_venv, _load_python_path, select_file.
- Add missing DocStrings.
- Add PlaceholderText using translations.
- Add TooltipText using translations.
- Improve file filter on QFileDialog.
- Remove unused import.
- PEP8 Fixes.
- Lint compliant.

Brancho:  Fix/project properties   :package: