WIP: Change directions shortcuts with a RTL language

Before this commit, the "forward" and "back" accelerators are hardcoded in the code to be, respectively, "right" and "left". This does not change under different environments, neither it is configurable in nautilus.

When using a RTL language it does not seem natural that the "forward" shortcut is the right direction and the "back" shortcut is the left direction.

This commit detects if the current GTK language is RTL and reverse the currently harcoded directions for "forward" to be "left" and "back" to be "right".

Fixes #1430 (closed)

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