Fix color profiles being assigned to screens by indexes

Before this commit, Krita assigned color profiles to screens
based on screen indexes, which apparently can change
(see the bug report). This commit introduces "screen string
identifier" which is a concatenation of multiple screen
properties like manufacturer, serial number, resolution etc.
(Note that especially on Windows, any of those
properties might be empty strings).

BUG:407498
CCBUG:412608
CCBUG:412943