FemApp intermediate updates

Lots of intermediate updates have been implemented, as they were needed before integrating the solver component 👌
Updates include; solver library moved to .NET Standard 2.1, added LBCs node labeling to mesher for LBCs application, switched from Unity's JsonUtility to Newtonsoft's JsonConvert, file structure made more efficient, UI escapes from mobile screen notches, last but not least: added 2-fingers navigation 🥳