Added shortcut to the button 'Cleanup entries'
Update CHANGELOG and ABOUT
Fix error when setting unset field in source panel

When the user tries to set a previously unset field of an entry via the
source panel of the entry editor, the program complains with an error
message and refuses to store the field values.
This change fixes that behaviour.
Merge pull request #19 from dahlheim/fix-source-edit

Fix error when setting unset field in source panel
Merge pull request #17 from eduardogreco/master

Solved the task #850 Keyboard shortcut for 'Cleanup entries'