This kinda reverts commit 6aebd30de1 ("app: remove
icon sizing preferences"), except that the code base is different enough since
this old commit was mainly for GIMP 2.10.x.
In any case, after initially thinking that GTK+3 handling for high density
display would be enough, we finally decide that adding back a Preferences-wide
setting for overriding the theme-set icon size is a good idea (additionally to
GTK+3 automatic support).
The base idea for removing the feature was that GTK+3 has high density display
support, through the "scale factor". Typically a high density display will
normally be set as using a ×2 scale factor so all icons will be double size.
Unfortunately it turns out it's not enough.
For instance, on very small screen estate, even with a scale factor of 1, if the
theme sets 24px toolbox icons, it may still take too much space.
Oppositely on huge screens, even with ×2 factor scale detected by the OS, the
icons may still feel too small (this is possibly what happens with #7023).
Furthermore there is also a matter of taste. Some people like small icons even
when they have the space. Others may want bigger icons, easy to click on.
Finally you can like a theme for its color scheme for instance, but it may not
have the icon size you want. Right now, we'd need to duplicate every theme in
small or bigger size. Instead of doing so, let's just have this global setting
overriding the theme rules.
Comparison with the 2.10 implementation:
- We still provide 4 sizes: small, medium, large and huge.
- We don't have the "Guess ideal size" setting anymore. Instead this is now a
mix of the GTK+3 scale factor logic and the theme-set or custom size. I.e.
that on a high density display with ×2 scale factor, we could have toolbox
icons up to 96 pixels (48×2)!
- We now try to have less custom code in widgets as we append the CSS rules to
the theme (similar to what we were already doing for dark theme or icon
variants). What happens in widget code is mostly to connect to changes in
themes and redraw the widgets which need to be.
- The custom size will now affect: toolbox icons, the FG/BG editor widget (in
both the toolbox and the color dockable), dockable tab icons, the main
dockable buttons, eye and lock header icons in item tree views, eye and lock
cell icons in the item lists.
There are still a bunch of areas where it is not taken into account, such as
plug-ins, and various dialogs, but even in custom-made interface in dockables.
Ultimately it might be interesting to have a way to sync more buttons and
widgets to a global size settings.
Lastly, I fixed a bunch of existing bugs where we were updating icon sizes with
gtk_image_set_from_icon_name() using the const icon name taken from
gtk_image_get_icon_name(). As this was reusing the same string pointer, we were
ending with freeing the icon name.
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