Improve file explorer

[Rob4001]

This is both in the styling sense and functionality:

• [x] Make the style look nicer
• [ ] Implement right click menus for saving, importing etc
• [x] Make it resizable
• [x] Make it watch source folder for changes

[joerick]

Just one thing remaining on this, which is right-click to delete, but that has it's own issue - #30, so closing this as fixed.