#39 Improve const-correctness

[Markus Elfring]

Would you like to apply the advices from the article
"http://en.wikipedia.org/wiki/Const_correctness" to
your sources?
I suggest to add the key word "const" to the type
specifiers for parameters like the following.
- file (function "expand_tilda")

http://cvs.sourceforge.net/viewcvs.py/fbpanel/fbpanel/misc.h?rev=1.14&view=markup

- type (function "plugin_load")

http://cvs.sourceforge.net/viewcvs.py/fbpanel/fbpanel/plugin.h?rev=1.12&view=markup

[Markus Elfring]

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How do you think about to clarify any APIs as a documentation aid?