theme/stylesheet quirks

Stylesheets that don't have a disporder set (for example because a plugin added them), are ignored by global.php. That makes disporder absolutely mandatory, and unnecessarily breaks plugin compatibility; for that reason I've changed update_theme_stylesheet_list() to update disporder by default.

(above disporder issue came up here: http://community.mybb.com/thread-87399-post-1096300.html#pid1096300 but it's not limited to that)

Attaching stylesheets to files vs. colors is a bit odd; it's in the same data structure (so essentially, a color is a file too). In 1.8 files must contain '.php', a restriction that wasn't there in 1.6 (technically a regression, although it should matter little). However this was also a problem with the stylesheet properties form; you could add entries without .php, it was saved too, just no longer displayed anywhere and subsequently silently forgotten if you went back into the properties.

So there are several possibilities:
- check for .php extension in the form and raise proper errors if it's missing.
- allow any filename (as 1.6 used to) and decide color vs. file based on whether there is a color by that name.
- change the datastructure to treat files and colors as separate entities in the first place

I went with the allow any filename idea, because that didn't require extensive changes.

While it's technically possible to attach to both color and file at the same time (giving specific colors to specific files?), this isn't currently allowed by the frontend so I didn't change that and kept the original behaviour that a color attachment overrides any file attachment, at least, as far as the frontend is concerned...

Also removed some useless code ($plural = s doesn't work for all languages and didn't work due to a broken if() anyway).