Bug 490999

First let me mention that I appreciate quite a lot the design and overall usability of Skanpage.

I would like however to make a couple (or maybe more) suggestions on how to improve the "Export PDF" dialog.

The current issues are:
1. right now, one is expected to edit both the title and the filename to make the document unique;
2. directory path and filename are edited using a single box, while in general one saves multiple documents on the same directory (i.e. the directory mostly changes once per session, the filename changes always);
3. the `file://` prefix is redundant and confusing.

Possible solutions / proposals:
1. by default, derive the filename from the title, so that editing the title is sufficient to create a custom filename;
1a. yet, allow to edit the filename manually, maybe there could be an "auto" mode where the filename is derived from the title and "greyed out", plus an option to make it editable.
2. make directory and filename two separate boxes (I guess this could go well together with 1a);
3. either (i) use a drop down menu with the supported protocols, defaulting to `file://`, or (ii) hide the protocol when, and only when, it's `file://`.

I hope these suggestions can be helpful and thank you in advance for taking them (potentially) in consideration.