Reorganize static assets (stylesheets, scripts, images)

[robocoder]

Relocate static assets (e.g., images, stylesheets, and scripts) to the plugin's top level folder, instead of in templates, e.g.,

plugins/NAME/css/
plugins/NAME/js/
plugins/NAME/images/
plugins/NAME/templates/

Similarly, move themes/default into a core plugin, e.g.,

plugins/CoreTheme/css/
plugins/CoreTheme/js/
plugins/CoreTheme/images/
plugins/CoreTheme/templates/

Rationale:

• uniformity (lowers the learning curve)
• may make it easier to replace content at runtime (see Build "theming" feature in Piwik. #5597 theming)

[robocoder]

CoreTheme may be a misnomer as it isn't a self-contained theme. Let's go with CoreUI instead.

[mattab]

Maybe this could be bundled in the main CoreHome plugin? It would be good to minimize number of plugins :)

[robocoder]

(In [3187]) refs #1385

[mattab]

Moving requirements into "theming" as it is still to be defined exactly how it's going to work (or if we will implement it at all, probably just a simpler version) #177