ImageGraph: graphs should display % values

[JulienM]

Follow-up on #2704 4th point Pie graph % display, comment:ticket:2704:18 and comment:ticket:2704:19.

See attached file for POC.

Need to define which graph type are concerned and how the percentage values should be displayed.

[JulienM]

(In [5582]) * fixes #2706, #2828, #2704, refs #1721, #2637, #2711, #2318, #71 : horizontal static graph implemented

• fixes #2788, refs #2670, #1721, #2637, #2711 : default graph type logic moved to ImageGraph API, improved date/period logic, new parameter graphs_default_period_to_plot_when_period_range
• fixes #2704, #2804, refs #1721 : pChart updated to 2.1.3, pChart code removed from Piwik code, OOP refactoring, support for unifont.ttf if present in ImageGraph/fonts, testAllSizes now uses report metadata ImageGraphUrl
• refs #71 : space between report title and report table reduced to avoid page overflow
• refs #2829 : TODO display percentages
• r5544, r5547, r5549 merged