Make the Lua plugin loader safe for global use

It would break when another plugin had the same
module name.
Load C plugins with local linkage

C extensions can bring arbitrary symbols into the global symbol table
which is then shared with all other extensions and the main binary.
This is a recipe for symbol clashes which are hard to debug. Using
G_MODULE_BIND_LOCAL provides proper isolation and makes
applications more robust.

Based on a patch by Thomas Martitz

https://bugzilla.gnome.org/show_bug.cgi?id=740823
Test that C plugins have local linkage

Otherwise two symbols from different plugins could
share the same global reference.

Based on a patch by Thomas Martitz

https://bugzilla.gnome.org/show_bug.cgi?id=740823
Fix the test suite to work on other operating systems

Change the warnings so they can be checked properly in
the tests without relying on OS specific messages.

https://bugzilla.gnome.org/show_bug.cgi?id=727380
Fix the docs
Check the docs for errors during make check

This uses the provided gtkdoc-check program.
Fix cross references in docs

This approach is copied from Gtk+.
Silence `make demo`
Add po/libpeas.pot to .gitignore

Should be safe given gedit's .gitignore also does this.