Add .gitignore

Combined Android, JetBrains, Linux and Windows .gitignore templates.
Remove tracked files matching .gitignore

These files are generated and we do not want them in the repository.
Introduce Gradle and adapt to new Android layout
Regenerate .data files in new project setup
Unignore .idea dir partially and add its content

This allows sharing of e.g. code style, which makes code formatting a lot more
useful. Some files within .idea are still ignored according to
https://intellij-support.jetbrains.com/entries/23393067.
Ignore some locally changing files inside .idea/

Android Studio and IntelliJ both make their own changes to the now ignored
files making it infeasible to keep them tracked in our VCS.
Add auto generated changes to code style settings

This is the way IntelliJ and Android Studio seem to want to have the settings
XML file, so we'll let them.
Merge branch 'fix-project-setup'