Complete move of graphing abilities to MantisGraph

The move of the graphing abilities from the core into a new MantisGraph
plugin wasn't fully completed. This large patch completes the move by
ripping out remaining parts of graphing support from the core, and
moving them into the plugin.

Some of the things that have changed as a result of this commit:

1. Deprecated the graph_font configuration option in the core and added
   it back to the plugin configuration. This time there is a nicer UI for
   selecting the font from within the plugin configuration screen.

2. Deprecated the graph_colors option from the core and disabled this
   functionality in the plugin. This will obviously require a lot more work
   (outside the scope of what I can do at the moment) so that there is a
   better way for the user to select colours via the UI configuration page.
   Additionally, this option shouldn't just be restrictied to JpGraph.
   Graph colours need to be reimplemented properly at a later stage.

3. Deprecated the default_graph_type option from the core and disabled
   this functionality in the plugin. This was a very cryptic non-documented
   option that set the default plot type on bug_graph_page. If this is to
   be reimplemented, it needs to be done so properly so that it isn't
   cryptic and unusable. There is no real loss of functionality with the
   removal of this option.

4. Reordered the layout of configuration options in the plugin
   configuration page so similar options are grouped together. It also
   looks much nicer now thanks to some changes to strings, and the
   placement of comments on a second line in small text.

5. Moved all strings used solely by MantisGraph into the plugin. The
   original strings have been removed from the core as they aren't used
   there.

6. Removed references to the graph related options from the
   Configuration section of the Administrator Guide.

7. Moved class.period.php from the core to the MantisGraph plugin as it
   currently isn't used elsewhere within Mantis. Really this class should
   be part of the core of Mantis (under
   core/classes/MantisPeriod.class.php) but it'll need some more work
   before it can be used in other parts of Mantis. For instance, we don't
   use this class for date filters. More importantly, PHP has a built-in
   class called DatePeriod (and DateInterval) that appear to be better
   choices for future work with date periods.

8. Added a UI field in the plugin configuration page that is only
   accessible to users with access level >= admin_site_threshold to set
   what once used to be $g_jpgraph_path.