support reloading of RPKs if changed on disk

[mistafunk]

support reloading RPKs if they have been re-exported from CE while Houdini is running
additionally: prevent resolveMap->getX crashes if RPK keeps name but e.g. the cgb file inside are renamed

[MattB3D]

My thoughts:
1: Reloading at Houdini restart (works)
2: Reloading atfter browsing for one (works)
3: Reload RPK button (would be nice!)

I'm not sure if we need automatic change detection live in Houdini .. I'd skip that, as typically the Houdini artist is probably the same person that just edited the RPK.

[mistafunk]

the plan is to automatically reload RPKs if they change on disk (and update the cache as well), i.e. no reload button

[mistafunk]

implemented RPK reloading based on file modification time stamp (still on the branch). currently, the reload and recook is only triggered if a pldAssign parameter is changed (or if a parameter dropdown is opened).

still pondering if we need an actual reload button...
also, we could install actual file listener which would reload (maybe even recook) fully automatically (but might be undesired for heavy scenes).

[mistafunk]

another open design issue: currently, choosing an RPK (or reloading it) resets the other three main params (start rule, rule file, style). is that wise? on the other hand, one could ensure these three values with an "attribute create" node before pldGenerate...

[mistafunk]

renamed issue to deal with the actual reloading of RPKs... let's deal with automatic reloading in a separate issue

[mistafunk]

merged to master