Optimize current state of app

[florianfendt]

Making a little list here what the next development steps will be.

Sidenote: BLOCK for introducing new features until all this is done:

• All tabPanels need to be properly reviewed and possibly refactored:

• learners: too complicated, introduce reactiveValues
• train/predict: input handling is not great when data, is missing or prediction failed
• preprocessing: undo-button and related with that is handling and assignment of data$data needs checking
• task: again data$data!
• other tabs: imho ok but see below.

• This somehow is related to the second checkbox but I want to stress it here separately so we really do this properly: Review everything for style guide, function usage:

• variable names: Maybe we need to deviate from Bernd's style guide here because we get problems with . when we use the variable in a java-expression. Right now we use both _ and ., which is bad.
  @Coorsaa let's agree on a solution for this on Monday.
• reactiveValues and reactive-functions need to be used in correct context consistently
• stringi for string-operations everywhere
• checkmate should be removed. We only use it in 2/3 places and should be done with needy-functions within shiny

• Find a better design for the UI and apply everywhere. Ideas for now:

• Use flexdashboard to move sidebar to top. Our section specific inputs that are now in our
  makeSidebar function can then go in a sidebarPanel. Maybe we can even use the one from shiny dashboard for that?!
• Colors, button style, ... (I think the radio buttons are super ugly, dunno if thats only me though 😄 )