do a proper loadbang for canvas fields (needs more testing)

(differs slightly from mine in that the '-' character is used to mean
 a wildcard template)

from pd-l2ork: 1e654625d71b6c0efae614cfa428e8cfae6e9b1e

from pd-l2ork: 640d3c53ad8cd592b1d757b988e27d3ada2b1779

from pd-l2ork: 9e88d7ac62c7c4a2717efe7953ac2165e1c5b816

(this should change modes correctly and address all the failed hunks in the previous porting attempt)

from pd-l2ork: 93a4a48649f3ed2193bad07fb7ea6ecb07c19760

from pd-l2ork: 7e3f0fd109b3c726acfdb75a19f4c83a838670ce

from pd-l2ork: fb109233c5b8c2ed4bbc5d0f7598cff4e2ed7a10

from pd-l2ork: f37aeb038f4bd20b2ac74c9c2d81f75f72162216

from pd-l2ork: f350d1c1fa5ec054259a2b2fc37e07fb13eb7e66

from pd-l2ork: f65d6bb5a6c7eb762b90ecef0a78c84fe7195da9

from pd-l2ork: 385ceb89ca38b34ca0483d5b79d6c18b29b21805