files: Implement selections using shift key

Previously only the Control allowed to add multiple entries to the current selection. This allows the use of Shift to select a range of entries. Shift + Control combinations are also supported now, similar to nautilus, thunar, etc.

Type of change

Please tick the relevant option by putting an X inside the bracket

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Checklist:

• [x] My code follows the guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] All current GitHub actions pass
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have checked my code and corrected any misspellings
• [x] I have read the CONTRIBUTING document