Allow users to download songs

[dexterlb]

This PR adds support for downloading songs in three different formats:

• Plain text
• The UG internal format
• ChordPro

The module that converts UG tabs to ChordPro is self-contained (in chordpro.py). I've used this patch for a couple of months now and ironed out most of its bugs - I can confirm that it works well for almost all of the songs I've tried. The ChordPro format is supported by multiple open-source programs and phone apps, and the official chordpro tool allows creating pretty PDF songbooks from collections of ChordPro files, and also stuff like transposing for piano and different notations.

I'm guessing that the plain text export would also be useful for a number of people who want to just be able to quickly view songs offline.

I've tried to keep the changes to the freetar UI as minimal as possible, and I'm open to any modification suggestions.

[mmatthe]

I can confirm this PR works very very well and I would love to see this merged. Now, there are two features in two distinct branches (download and pagination) which are both very good to have but I need to choose between one of them for now.