Mus 94 refactor piano view

Please do not change the .gitignore! I played around at least 10 hours to find out the best solution for about 100 students using shared settings and different platforms. There is an instruction about the project setup in the Github Wiki you should have read in the first place. Now please do the following:
* undo the changes to .gitignore and commit it
* close Android Studio!
* execute in Terminal in project root (all written code should be committed, or will be lost)
  * "git reset --hard" should reset the iml-files and misc.xml
  * "git clean -xdf" should delete some hidden settings files
* proceed last 3 steps: https://github.com/Catrobat/Musicdroid/wiki

PS: in the current version of Android Studio Google fixed a bug. So maybe there will be an updated version of these settings without the iml-files soon.