Preferences

[aleksandar-stefanovic]

Added preferences dialog

[donadigo]

Thanks for contributing again! We don't need static instances in preferences dialog, since it's only used in one class, so you can just do in constructor of ToolBar class:
preferences_dialog = new PreferencesDialog ();

and then show it:
preferences_dialog.show_all ();

Creating methods like "get_general_box" isn't really needed and doesn't make sense here. The contents of get_general_box method should be in the constructor of the PreferencesDialog.

Although I like when someone contributes to my project, I don't want any pull requests submited now, just because there are still commits with huge diffs, the project is very young and often you will need to resolve conflicts before I can do a review, but if it's not a problem for you, feel free to sumbit them :)

[aleksandar-stefanovic]

I thought that there will be a complex layout in the future, so we could already split it into multiple methods, for readability (esp. considering possible multiple tabs).

All right, I just want to be helpful, if it's creating you a lot of work, I won't pull more requests, until the projects gets stable... regarding merging, it's not an issue for me, I don't think that I'm working on the important parts that would overlap, and if it happens, it usually isn't a big conflict...