allow default iemgui colors to inherit colors from GUI presets

See merge request jwilkes/purr-data!763

As discussed on the mailing list, it's useful to allow symbol
inputs (other than bang) on pddplink's inlet, to make it easier to
invoke pddplink on a variable symbol (filename, patch, URL) value.

Note that for backward compatibility, a dummy filename argument is still
needed. Please check the updated help patch for details and an example.

Due to the usage of "keyup" as a trigger for the search, the aucomplete
window disappears before the user can take an action - making it
useless. That's not optimal, but it's a quick fix.

Core documentation (inside pd-l2ork/doc/...) should be ranked before
external libraries. Add a multiplier to increase the score of files
inside that folder.

Chosen priority: title > keywords > description. The chosen weights
(boost parameters) were chosen arbitrarily based on manual iterations
and could be improved.

Word expansion was enabled in title and keywords fields to allow
matching of parts of a word.

This adds a canvas member variable and the necessary logic so that
loading and saving zoom can be enabled and disabled in a canvas-local
fashion via `declare -zoom`, overriding the global sys_zoom flag.

An update of the declare help patch is included. It adds a new
Patch_local subpatch which explains the new option.

The save abstraction operation must use the basename without the extension for
the object name. This regression was introduced in rev. 752294af, the present
commit corrects this while still preventing string overflows.