Add new tab with text area.
add three buttons for Open, Save and Apply

Open and Save open a file dialog, but so far without any functionality.
Open file and write content into text field.
Parse the input string and reset the simulation.
Implement basic YAML parser (using SnakeYAML).
Write initialization text area to file.
Add confirmation dialog if file exists.
Start file chooser in "input" subdirectory.
New YAML setting for pair of particles.

Add YamlParticle class to create new particles from a YAML description.
New YAML settings for two stream instability.

Add YamlParticleStream class to create stream of particles.
automatically apply settings after opening a new file.
Small cosmetic changes

- remove unnecessary import
- change order of setting particle properties and adding it to
  the list of particles (the order does not really matter)
Merge pull request #79 from aipp/file

Load initial settings from a YAML file