Play sounds using javafx media.

Doesn't work on my Ubuntu, needs to be investigate (no error and no
sound).
Added PoC for UI tests.

Moved sound resources
Simplified the documents load process

Removed buttons enable/disable during the pdf document load. Now the
selection table is filled as soon as the documents are selected and pdf
metadata (like pages etc) are populated asynchronously, allowing the
user to work without having to wait that all the docs are loaded.