Display: Support fractional scaling if enabled

[leolost2605]

This PR brings multiple improvements:

• We allow scaling per display if supported
• We allow fractional scaling per display if supported or rather enabled

Fractional scaling is still disabled by default so the fractional scales will not show up in the dropdown by default.
To enable it add 'scale-monitor-framebuffer' to the mutter experimental settings

Fixes #117

[danirabbit]

Works as expected! Nice work! Awesome to see support for mixed DPI settings working 👏