Add four new preferences

Add four new preferences

This commit adds four new preferences:
* Whether to use saved window positions also for new windows (in case saved
window positions are enabled),
* Whether to enable navigation by left or middle click on image,
* Ability to set keyboard scrolling step multiplier,
* Whether list directory view uses single click to enter a directory or
GTK+ activation default (which is currently a double click).

Default values of all these preferences have been set in a such way to not
cause any changes in behavior for existing users.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>