Merge rendering modules

[Guite]

This change does not affect the smarty subclasses, but only the backend modules and administration stuff.

1. Merge the pnRender and pnForm modules into the Theme module.
- system/theme/plugins/ <-- pnRender und Theme plugins
- system/theme/plugins/forms/ <-- pnForm plugins

[Guite]

Having the pnForm plugins in a separate directory is required. Otherwise people could try to use pnform plugins in "normal" Renderer or Theme templates.

[Landseer]

(In [25061]) first part of merging pnRender into Theme, this refs #386, configuration pages have to be merged
renamed pnTemp to zkTemp
renamed pnRender_* folders to module_*
renamed Xanthia_* folders to theme_*
upgrade not yet supported, you need to reinstall 2.0.0! Please report any issues as comments to ticket #386

[Landseer]

(In [25065]) more renaming, moved configuration from pnRender to Theme module, this refs #386

[Landseer]

(In [25066]) replace Xanthia_Config with theme_config, refs #386

[Landseer]

(In [25074]) removing obsolete pnRender module, this refs #386 and closes #16

[Landseer]

(In [25127]) moved pnForm to Theme, split up pnForm.php into separate files for each class in includes/forms/, removed pnForm module, this closes #386 and refs #18 (more renaming is needed here)

[Guite]

See also #1545