Settings for changing the scaling manually

Even when an on-screen-keyboard will adapt to most screens well by default, there will likely always be people who would like it to be smaller or larger, so they can, for example:

• Type more comfortably.
• Have more screen-space available for apps.
• Comfortably use it on a distant, relatively small/large screen (for example, when a computer is used with a gamepad with a touchpad, or with another type of handheld pointing-device).

So it would be useful to have a setting for adjusting the size of the on-screen-keyboard easily. GNOME Settings and Phosh Mobile Settings seem to be appropriate places for that.

I think that a multiplier for vertical and horizontal orientation, 1 each for both internal screen and external screen (so 4 settings), going from about 0.25x to about 2.00x (probably further limited to about 67% of the screen-height) should be good enough for most cases.

For this to work properly, it is necessary to adjust the layouts to different aspect-ratios automatically. However, even before Squeekboard can do that, having the settings for the internal screen could be useful to work around scaling-issues on various devices.