Add support for linear histogram

Add a config option to allow switching between liner and
logarithmic histrogram rendering

Signed-off-by: Daniel P. Berrange <dan@berrange.com>
Refresh translations from transifex

Signed-off-by: Daniel P. Berrange <dan@berrange.com>
Add note about 'h' key for histrogram mode toggle

Signed-off-by: Daniel P. Berrange <dan@berrange.com>
Ensure every event is emitted in main thread

Ensure that the pixbuf loader emits events in the main
thread, allowing all calls to gdk threads enter/leave
to be removed

Signed-off-by: Daniel P. Berrange <dan@berrange.com>