Protect private definitions by the GEANY_PRIVATE macro in headers

In addition, this patch defines the GEANY_PRIVATE macro for the tag
manager library where it wasn't defined before, removes 2 tag manager
headers from distribution as they are not needed by plugins and
in the tag manager changes the docstrings to ordinary comments for
private definitions.
Fix doxygen warnings

Make documents_array public and accessible by doxygen and describe all
parameters of tm_source_file_new().
Define the GEANY_PRIVATE macro for tag manager in makefile.win32
Add all headers to libtagmanager_a_SOURCES in tag manager makefile

So they become part of tarball.
Merge pull request #351 from techee/guards

Protect private definitions by the GEANY_PRIVATE macro in headers
Small update of German translation
Rename internal plugin_init() to avoid confusion with plugins entry point
Make File Browser use explorer as default open command on Windows
Set -O2 compiler flag by default for gcc

But only if no other optimization flags could be found.
Merge pull request #362 from ntrel/fb-cmd

Make File Browser use explorer as default open command on Windows
Always set project description and file patterns in the dialog

Set them also when they are NULL in the project (which happens when new
project is created) because otherwise there are left-over values from
the previous project.
Merge pull request #363 from techee/prj_dialog

Always set project description and file patterns in the dialog
Make Next/Previous Message keybindings work without a document open
Recommend Glade 3.8.5, add link for fishman ctags fork
Introduce GEANY_EXPORT as G_MODULE_EXPORT wrapper

The wrapper is because G_MODULE_EXPORT can't be used if/when we want to
switch to hidden-by-default visibility and only export explicit functions
because G_MODULE_EXPORT is always empty on Linux.

Change glade callbacks to use GEANY_EXPORT. Also remove it from the declaraitons
so that it's only at the definition.
api export: Dynamic export of document functions
api export: Dynamic export of scintilla functions
api export: Dynamic export of sci_*  functions
api export: Dynamic export of templates_get_template_fileheader function
api export: Dynamic export of utils functions