move PDF sources into "manuals" subfolder and change how user data folder is resolved

[psifertex]

Current code only really works well from the plugin folder, not the plugin manager, PR incoming to solve this

[amtal]

Post-coffee thought. Add a "DROOG//Open PDFs Folder...", have it invalidate the cache to reduce restarts, have it auto-create appropriate endianness/size-agnostic arch name if called from a BinaryView.

Then location could be anywhere. (I prefer in-repo, that way can pre-create folders and version control PDFs in forks.)

[psifertex]

I'd still recommend having it look for things in the user-folder as well. Otherwise if the user uninstalls the plugin they might lose their curated PDFs they've added. Looking in both places (much like BN includes some types in the default install path but the user can add new ones in their user folder) seems like the best of both worlds.