Move the PluginEntry class which is defined within the cpp file into the

Akonadi namespace, as it clashes with a class of the same name in
kdelibs (in the plugin loading code) on OSX (at least). The effect
is that the wrong symbol is found at object instantiation time and
all hell breaks loose, including gdb giving up the ghost.

This is the second time we've run into this, luckily this time
Volker and me remembered the problem so it didn't take days to
find, like last time ...

svn path=/branches/work/akonadi-ports/kdepimlibs/; revision=1010100