Use default values instead of placeholders ( or remove placeholders )

[rodica-andronache]

This is coming from me, as I've always found the options using placeholders confusing.
Right now, for the majority of options, we are using placeholders ( for e.g. for the Minor Grid Lines it's 0 ), and I think that can create confusion. I have the feeling that's the current value for that field. I think we should either have that as a default value or not have it at all, as it doesn't help, from my point of view.

Moved from here #418

[DanyCaissy]

We can do this: Leave the placeholders as they are, but ALSO the default value should be the same as the placeholder (instead of empty)

This way the user will only see the placeholder if he erases the value.

[DanyCaissy]

Note: Please ensure pagination is defaulted to 10 as well, as per this ticket: #660