Fix multimonitor window size restoration

The size saving and restoring code saves width and height values under a
config key that changes based on the active screens, the name of the
current screen, and the resolution of the current screen. This suffers
from a few issues:
- Due to https://bugreports.qt.io/browse/QTBUG-50788, what counts as the
  current screen is volatile at runtime, and hence data gets saved to
  and read from different config file keys.
- Including the screen resolution of the current screen only makes sense
  for single-screen setups anyway as it's just random data causing
  entropy for multi-screen setups.

Both of these issues contribute to the problem of window size being
restored incorrectly for multi-screen setups.

This commit fixes the issue by no longer including current-screen-based
data in the config file keys for multi-screen setups; just including the
current set of screens is good enough and it's non-volatile within the
scope of what we care about.

BUG: 460260
FIXED-IN: 5.104
(cherry picked from commit fc46a221)