Introduce new theming system

This extends current templating system to another dimension,
bringing the possibility to have multiple themes generated
at once. The purpose is to be able to switch between multiple
designs, from PC to mobile, classic vs. flat view, slideshow, etc.

For the moment, only the classic theme is available.

Rules:
 * for switching between themes, place a link inside your template
   manually - it's not a cgg concern, only your theming infrastructure
 * don't forget to define different file names for index and album
   pages across different themes
 * it's recommended to keep default theme named as "index.*" to avoid
   showing directory listing on webserver

Notes:
 * some TODOs will be fixed when we have new theme using these advanced
   features (e.g. pictures in album pages)

 * TODO: introduce tag/block conditional system, allow custom user
         defines for each theme/page

 * TODO: deprecate <show_go_up>, <show_exif_table> and <border style>
         in favor of conditionals (these tags belong to theming)

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