fixed template_from_string when the template includes or extends other ones
optimized the filesystem loader

* added a cache for templates that do not exist
* sped up Filesystem::exists() by avoiding the creation of exceptions
merged branch fabpot/filesystem-optimizations (PR #1155)

This PR was merged into the master branch.

Discussion
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optimized the filesystem loader

* added a cache for templates that do not exist
* sped up Filesystem::exists() by avoiding the creation of exceptions

closes #1096

Commits
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d2b97b4 optimized the filesystem loader
merged branch fabpot/complex-template_from_string (PR #1154)

This PR was merged into the master branch.

Discussion
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fixed template_from_string when the template includes or extends other ones

This should fix #1134

Commits
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1b5bf91 fixed template_from_string when the template includes or extends other ones