Display icons for 3rd-party blocktypes plugins

Bug 1510421

Defines a new static PluginBlocktype method, get_css_icon(), which
fetches the name of the CSS icon to use for this blocktype. It returns
false by default, which tells the theme to "fall back" to the old
thumbnail.png instead. 3rd-party plugins can override this to
specify a particular icon to use.

All the core blocktypes have been refactored to extend
MaharaCoreBlocktype, which uses the blocktype name as the name
of the CSS icon to use. I also deprecated the "SystemBlocktype"
class while I was at it.

PluginBlocktype::get_blocktypes_for_category() now returns both
the results of get_css_icon() and the thumbnail.png path, so that
themes can decide which they want to use. (And of course
thumbnail.png is served via thumbnail.php, so 3rd party themes
can provide their own custom image files if they wish.)

behatnotneeded: Requires installing third-party plugins to test

Change-Id: Idb1ecfc7b21175913708e695788906c11133b0c0