Preserve order of script/stylesheet includes

[sgfost]

Changes the collecting of an element's includes to use dicts to preserve the order specified by the element and (and retain the prevention duplicates). As noted in #24, this is needed when a module has dependencies that rely on others being loaded first.

Alternatively, using lists may make it slightly more clear what is happening but would require the additional step of removing duplicates

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resolves #24

[rht]

Thank you for fixing the long-standing bug!

[sgfost]

Sure thing, thanks for your work on mesa