"Open Map Scripts" button

[garakmon]

(copy of yenatch/pretmap#70)

Adds a button to open a map's scripts in a the system default text editor.

Should close Issue #4

demo:

[huderlem]

Rather than using those different commands for different OS's, we should use Qt's built-in functionality:

QString path = QDir::cleanPath(editor->project->root + QDir::separator() + "data/maps/" + editor->map->name + "/scripts.inc");
QDesktopServices::openUrl(QUrl(path));

Plus, the Windows command didn't work for me.