Fix race condition for display

The external display takes a little while to register, so this commit
makes `think-dock` check repeatedly for an external display until one is
found or 5 seconds have elapsed.
Update translations
Refactor external display checking into function

Additionally, don't sleep before 1 second before checking for the
external display.
Make get_external_display return correct exit code
Remove get_external_display function
Add support for other X screen naming style

X screen names can be of the form `(:0)` or `(:0.0)`; add support for
the latter style.
Changelog entry
Merge remote-tracking branch 'jturner314/fix-user-detection'
Merge branch 'master' into display-race
Use C-Style arithmetic
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