desktop: Expand variables

2f202ab6 removed expansion of variables, but @bindir@ still needed to be expanded.

This partially reverts that commit, without re-introducing Bugzilla related data.

Also the makefile rule for $(desktop_DATA) was using the same .in file as input for all the desktop outputs. Resulting in the same file instead of having the correct --components.

Fixes #131 (closed)

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