GTK3: CSS: use same font sizes as GTK2 did

If you want to customize this (on a per-user basis),
create file named ~/.config/darktable/darktable.css
with following content:

@import url("/usr/local/share/darktable/darktable.css");

* {
  font: Sans 10;
}
... <rest omitted> ...

That will bring font size to what it was before this commit.
GTK3: fix viewswitcher styles
Port imageio to bauhaus

This commit also tries to space & align the widgets in export in a
pleasant way.
Left align a label, forgotten in last commit
Bigger icon on rotate buttons
Align widgets in metadata and export

I am not sure that this is a good idea, long strings will harm the whole
GUI and not just their lib.
Lens IOP: fix alignment for some elements of lens tooltip
GTK3: viewswitcher: just use css, say no to markup
cache: start stupid re-implementation with big fat locks
cache: somewhat cleaner, slower, glib cache
cache: continue to break things in image_cache
cache: image_cache reworked to use new backend
cache: fix a couple of compile issues
cache: compile fixes
cache: port grouping
cache: port variables
cache: image_cache fixes
cache: exif.cc
cache: history and image_cache implementation
cache: port image.c