Add an option to disable click to advance

[ulteq]

This allows the user to decouple the pan modifier from the next image modifier.
However --disable_click_to_advance might not be the best name for the new option.

Closes: #411

Related with #310:

mouse button 1 (left-click hold) can be assigned freely, but the default behavior of short 'left click' (move to next image) seems to be not configurable; letting both 'left click' and perhaps also 'left double click' be configurable would come handy

[klesh]

I often myself trigger next_img when I try to pan the image...
That's very frustrating. So I think this option would be very helpful, don't you think?
What was wrong with this PR anyway?

[teleshoes]

+1

[LuciusMortified]

+1

[pepa65]

It would be way nicer if the single (mouse)button click can be configured separately in the buttons file. Maybe like: toggle_fullscreen s-1 (lower case 's' for "single") to bind [toggle_fullscreen] to single-left-click. That 's' would be instead of the S/C/1/4 modifiers, signifying a single click within the bounds of time set for that.

[Runeword]

+ 1