GUI Rework: reorganized folders and split app.js

Hi there!

This is the first pull request for the GUI refactoring I mentioned some days before. I started with reorganizing the folder structure and then split the very large app.js into seperate files. I am also using a module system for the gui core components. More modules will be created as the further refactoring goes on. Like one module per usecase.

The folder structure is a bit cleaner now, since third party libs like jquery or bootstrap, are in vendor now. Syncthings own assets and scripts are in folders, having the similiar name.

I wanted to file this pull request before continuing, to get the changes back into master asap. So I don't collide with the changes of someone else here.

Just let me know what you think. In case of doubt please feel free to ask me.

Thanks & Regards,
Dennis