Allow editing non-component files

[jonnor]

Right now one can edit graphs, components, and (fbp-)spec files.
However, very often in a non-trivial project components import some non-component code. Sometimes the majority of the logic is done there, and the components are mostly wrappers that makes this nicely accessible to plug functionality together as FBP graphs.
When Flowhub cannot view/edit such files then one often has to use ones favorite-editor instead, either creating a two-tier setup where component editing is done in $editor and only graph editing in done in Flowhub. Typically also loosing benefit of live/interactive editing...

Problem is extra bad because of lackluster local project support #397

[jonnor]

Also related to making component libraries #410 and making module to use in components #273