Add tooltip mentioning keys to shift buttons again.

Also, change them in all translations with the help of find and sed.
I am not entirely sure if this is correct for RTL languages like arabic,
but it does look similar to the other tooltips :)

Commands used:
find . -name "*.po" -print | xargs sed -i '{N
s/msgid "Shift Left"\n\(.*\)"/msgid "Shift Left [<<]"\n\1 [<<]"/g}'
find . -name "*.po" -print | xargs sed -i '{N
s/msgid "Shift Right"\n\(.*\)"/msgid "Shift Right [>>]"\n\1 [>>]"/g}'
Merge pull request #15 from stefanct/master

keyboard shifts again