This PR refactor the "inputs design system" by reverting the correlation
'$input-bg == $light == color-3', initially introduced by #120302.
Beside ensuring color-consistency, the former system aimed to simplify
palette edition by clarifying how color-3 was used (simply all the UI
elements...).
While the former system was responsive to user customization and
color-presets, it didn't necessarily delight everyone's discerning
taste, at least not with default settings/palette.
This commit enforces a classic "white with borders" design that's
independent from the color palette and doesn't adapt to color-presets.
The rationale behind this decision is that the need for non-white inputs
is "nonexistent" and exceptional cases should be addressed using the
SCSS editor.
As a workaround to ease edition for users that still wants to challenge
themselves with the creation of "not standard" palettes/designs, this
commit introduce a colorPicker option assigned to $input-bg.
The hope is that this new controller help users finding a "compromise
color" that could work with any color-presets.
task-3568806
closesodoo/odoo#139642
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>