Improved theming override

[Tumulte]

This is (almost) the part 2 of the previous ticket.

If the template override system is to be modified, I believe it'd be a great idea to allow multiple themes and a customizable override order :

That would allow someone to make small design enhancement/mods without having to build a whole theme (perfect for plugins).

You would have to add those lines in the ini :
#the main theme
theme_base=base

#optional themes, each one override the previous one - that overide the base theme
theme[0]=my_theme
#if someone has made a theme for devices with screens smaller than 320px :
theme[1]=phone_theme
# some plugins would require to have this line added to the ini
plugin_theme[2]=my_plugin_1

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That'd make theme and plugins way more flexible whith, at all time, a clean basic theme.