Use mouse forward/back button to change image

[jakar]

I find myself wanting to use these buttons out of habit, especially when thumbnail view isn't open. With the wheel, sometimes I'll accidentally skip over an image or two.

[jakar]

Please review this carefully. The mouse event handlers in ImageView don't check the button, so this change, as is, might affect those. One of the following might be necessary:

• Return true after calling on_action{Next,Previous}_triggered in MainWindow::eventFilter.
• Test event->button() in ImageView::mousePressEvent and so on.

However, it seems like this depends on the desired behavior of mouse cursor. So maybe neither is necessary.

[tsujan]

I'm not sure but this may interfere with the new annotation capability. Someone having a mouse with forward/backward buttons should test it. Can annotations be drawn with those buttons and without this patch? If yes, forward/backward buttons should be excluded from annotations first.