Always set project description and file patterns in the dialog

Set them also when they are NULL in the project (which happens when new
project is created) because otherwise there are left-over values from
the previous project.

To reproduce this issue, open the project dialog for an existing project which has the description and patterns
set. Then create a new project and open the project properties dialog - the values from the previous project
are still present even though the fields should be empty.
Please don't assign string literals to non-constant pointer, and more generally only use non-constant pointer for strings that should be freed.
I realize it means you probably need two separate variables here, but I think it's worth it.  Or use a ternary like
```C
gtk_text_buffer_set_text(buffer, p->description ? p->description : "", -1);
```