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

Test plan

1. Have one screen
2. Open KWrite
3. Resize its window
4. Close KWrite
5. Open KWrite -> last-used window size is restored
6. Attach a second screen
7. Open KWrite
8. Resize its window
9. Close KWrite
10. Open KWrite -> last-used window size for the multi-monitor setup is restored