Add save/open project function

Issue : None

Save the project into a Amapola Project File (.apf) :
- Action "File" menu "Save" or "Save as"
- Ctrl + S

Open an project file :
- Action "File" menu "Open"

Fix some bug :
- #31 Delete shall delete the text, not the subtitle;
- #27 Save and open subrip as utf8;
- #20 Save file with the correct extension;

Signed-off-by: juiice <decaen.julien@gmail.com>