Mark some string translatable and Add Turkish translation

[ghost]

This PR contains three commits.

The first commit marks some missing string translatable.

• "Select an item from the Library to open or create a new Sheet."
• "Preview"
• "Preview : "
• "Sheet Notes" and "Project Notes" from Notes.vala

The second commit fixes the LINGUAS file order. Also adds the nl file in LINGUAS which wasn't listed in it.

The last commit only adds Turkish translation.

[ghost]

@kmwallio Do I need to upload my translation to Poeditor.com?

[kmwallio]

Thanks for contributing! I'll upload the translation to Poeditor.