gui: force vert scrollbars on in module list to prevent jumpiness

without forcing the vertical scrollbars to be present at all times, the
modules can "jump sideways", this is particularly noticable in the left
module list, since it has the scrollbar appear on it's left side, moving
the module to the right to make room for it.
(cherry picked from commit 26261b3f375873834714a1e6f502bd3dd9186dc9)
gtkrc: cleanup scrollbars

first we leave a bit (2px) breathing room between a scrollbar and
neighboring widgets, so they don't get stuck together in an ugly
fashion.

second we darken an insensitive scrollbar button, so it doesn't
distract anymore.
(cherry picked from commit 99811d0872bd25c2da803fc967bb5540e6743533)
darktableconfig: restore default panel width to 300
(cherry picked from commit 31cb0bf894f4fb09182072885b947d2a90e2385c)
darktableconfig: default image_in_row to 5

on lower res displays having 7 images in a row by default is
a bit much, which results in fairly tiny thumbnails, which
aren't particularly useful.
(cherry picked from commit 06b111b36b0164b7263a8bf67fc4c04941f865ab)
tagging: don't glue buttons to treeview
(cherry picked from commit 134ce6259597ebccfe88c1cb066312a0af2c98d1)
tiff: explictly mention 32-bit is float
(cherry picked from commit 078712b0e7312073fc290a25f007bf1eca326680)