display-settings: Show scaling values using a combo-box when more than buttons limit

In case fractional scaling is enabled, depending on the resolution, mutter may advertise many fractional scaling values which are way more than MAX_SCALE_BUTTONS, making impossible to use higher fractional scaling values in high dpi setups.

So, in case scaling is possible, show them as buttons if they fit the buttons limit, otherwise fallback to a combo box that is consistent with the rest of the UI and will allow to use any valid scaling value.

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