Frames: allow absolute units for size and gap

As discussed in https://musescore.org/en/node/290637, this part bug, part feature request.

The size and "gap" of vertical frames, horizontal frames and text frames is measured in relative units (stave spaces / sp.). This means that when the user adjusts the scaling in Page Settings, frames grow and shink with the music. However, in many cases (perhaps most cases) it is actually desirable to keep the frame a fixed size regardless of changes to the music. This means there should be an option to set frame size and gap in absolute units (inches or millimeters, depending on the user's locale).

Frame margins are already measured in absolute units, which leads to strange behaviour when scaling and means that the frame can sometimes end up smaller than its own margins. To prevent this, the option to switch between relative and absolute units should probably affect frame size, margins and gap all together, rather than being adjustable for each individually.

Options to switch between relative and absolute units already exists for images and text inside a frame, so it is strange that the same option does not exist for the frame itself. The "size in stave space units" checkbox from the Image Inspector is a good model to use for this option in the Frame Inspector.