Don't switch the current tab on right-click events

This change makes sure to keep the active tab on right-click events to
make Nemo behave a little more akin to applications like Firefox or
Chromium. Considering that menu actions like "move to left", "move tab
right", and "close tab" were previously all based on the current tab,
this change is slightly more invasive than desirable since we have to
pass the target page number along to quite a few helper functions now.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>