Ensure polybar builds without optional dependencies

NBonaparterequested to merge
patrick96:no-feature-build into master

Created by: patrick96

The main goal of this was to make sure polybar can still build even when everything optional is turned off. I have added a new travis build configuration to do that. This new build would have caught the build failure caused by building without xrm.

This also fixes the underlying cause of the xrm build failure (missing include)

While working on this, I have also noticed that the xcb-xkb is optional, if we make the xkeyboard module optional

Fixes #1264

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