file.importProject: import an EffectCraft project into the open project

File > Import > EffectCraft Project (and .ecproj/.ecprojx given to File > Import)
merges the project into a folder named after the file, with every id shifted past
the open project's, in one undo step. Relative footage resolves next to the file;
the result lists comps, root comps, primaryComp and missing media.

offset_ids now shifts tracker Motion Targets and saturates instead of overflowing;
fix_next_id saturates too, so a damaged file can't overflow on open or import.
Menu entry translated in all four languages.

(cherry picked from commit 19c653fccc95133fb35e978a7f7f20a18c0e8c89)

(cherry picked from commit f4d36d11b0ccdfc56371a3bbd8d43d3b8281ee1b)
(cherry picked from commit c08135b2de8429693e723d065584465c459a61f9)

docs/parity-log.md

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+
+## Update: File ▸ Import ▸ EffectCraft Project
+
+- **Import a project into a project**, as After Effects imports an `.aep`:
+  `file.importProject {path}` (File ▸ Import ▸ EffectCraft Project..., and `.ecproj` /
+  `.ecprojx` files given to File ▸ Import) merges the project into a folder named after the
+  file. Every item, layer and property id moves past the open project's, so references
+  (precomps, parents, track mattes, Layer Control values, Essential Graphics, tracker
+  targets) stay intact. It's one undo step. Footage paths relative to the imported project
+  resolve next to it. The result lists the comps, the root comps, `primaryComp` (the only
+  root comp) and missing media.
+- Id shifting saturates instead of overflowing on damaged files, for both templates and
+  projects.