Library variables are not shown
More error messages
System-specific look and feel
Disabled column reordering + cursor listener
TreeTable only collapses when new function is called
Text area background color
Added jSplitPane
Interpreter exits when done
Merge pull request #89 from fabian1995/master

Merge -> undef var values
[enh][interpreter][gui] mark not initialized variables as undef in gui

* TODO: Arrays, Strings