redraw signal: simplify the locking logic somewhat.

and make it more responsive, i.e. don't forget redraws issued from the gui thread.
redraw: allow more redraws per second.

this makes dr mode less laggy.

maybe this is still the wrong place to do this, maybe import (main problem)
should just throttle the signal emission instead.
redraw: remove now obsolete flood stopping.
do nothing in dt_dev_read_history() if no modules are loaded

this avoids warnings like the following if history stack is discarded from within lighttable mode:
[dev_read_history] the module `sharpen' requested by image `IMG_7273.CR2' is not installed on this computer!
don't reset zoom when adding new instance

Conflicts:
	src/develop/develop.c

Conflict was caused by a whitespace anomaly.
develop: don't call any gui things in dt_dev_load_image

it's called by thumbnail creation and 'z' and export etc.