Modernize the code, remove clutter and optimize functionality

[xx-shitai-xx]

• [ ] Fix for loops to use for (auto var : vector)
• [x] Use shared_pointer
• [x] Code style cleanup
• [ ] Remove code duplication
• [ ] Re-organize project
• [ ] Break down class interconnections
• [x] Fix up includes
• [x] Switch to default member initialization
• [ ] Introduce proper namespaces

[xx-shitai-xx]

This is obviously still on-going and might never be completed :D

[xx-shitai-xx]

@LoneBoco Fill in with tasks (even those you've already compeled) and check them off the list. It's nice to keep track of the changes we've done. :)