Introduce support for specifying variables from X resources. (#2286)

This patch introduces a new 'set_from_resource' config directive which
allows defining a variable by retrieving its value from the X resource
database. This avoids having to configure a color scheme in multiple
files. The directive takes an additional fallback value which is used
in case the resource cannot be found or during config validation where
no X connection is available.

Furthermore, this patch includes the following changes:
- If the same variable is defined twice, we now properly overwrite the
  value of the assignment rather than inserting two variable definitions
  with the same key.
- We now depend on xcb-util-xrm to query the resource.
- Increase the buffer size for variable / resource assignments.

fixes #2130

travis/travis-base-ubuntu.Dockerfile

@@ -28,3 +28,14 @@ COPY debian/control /usr/src/i3-debian-packaging/control
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive mk-build-deps --install --remove --tool 'apt-get --no-install-recommends -y' /usr/src/i3-debian-packaging/control && \
     rm -rf /var/lib/apt/lists/*
+
+# Install xcb-util-xrm. This is a workaround until it is available in the
+# distribution packages.
+RUN apt-get update && \
+    DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends xutils-dev
+RUN git clone --recursive https://github.com/Airblader/xcb-util-xrm.git && \
+    cd xcb-util-xrm && \
+    ./autogen.sh --prefix=/usr && \
+    make && \
+    make install && \
+    cd -