Go to next/prev chapter plus configurable swipe directions

I added two new functionalities, both turned off by default.

First lets you jump to the next or the previous chapter. You have to enable double taps for this. By default it works on the right upper corner for next chapter, left upper corner for previous chapter.
I have added tap zones for this in defaults.lua.
Note that this will not jump ahead the last chapter or before the first chapter, so it is not going to the begin or the end of a file (unless on the first/last page a new chapter starts)

Second let's you decide in which direction swipes will be counted.
This is a response to my own issue #632.
You need to change one or both of the new settings (DCHANGE_WEST_SWIPE_TO_EAST/DCHANGE_EAST_SWIPE_TO_WEST) to true to see an effect.
do you mean `nil` when returning `""` in a function?