POS is loading all partners, not only customers

Hi,

Adding domain to res.partner model, so POS only loads "customers"
[IMP] stock: display company in location tree view
[point_of_sale] Domain functions to literals

Replaced domain functions with literals where there are only constants
Merge pull request #3803 from TheCloneMaster/patch-7

POS is loading all partners, not only customers
[FIX] hr_attendance: use float division for worked_hours field

Used to be rounded to integer while the field is a float.
Fixes #3526
[FIX] sale_layout: set 0 as default value for categ_sequence

Otherwise, in some cases, the categ_sequence can be set to null, which is not ordered the same way than 0, and you won't be able to sort lines with null and 0 as categ_sequence between each others
[IMP] When no stock moves, change whatever you want, otherwise no change at all for UoM.  #3440 Enhancement request richard-willowit

The default UoM can only be changed when there are no stock moves (or only cancelled ones), but to any category.
The purchase UoM can be changed whenever you want, but has to be in the same category as the default UoM.

Closes #3440

[IMP] When no stock moves, change whatever you want, otherwise no change at all for UoM.  #3440 Enhancement request richard-willowit

[FIX] Remove line

[FIX] Constraint should be applied when UoM or PO UoM is changed