widgets: add support for specifing size and position options via percentages of output dimensions

[PaideiaDilemma]

Adds the ability to specify size and position options as percentages of the output viewport.
E.g.

position = 10%, 10%
size = 10%,5%

This is allowed:

position = 10%, 10

Question: Do we want to support a px suffix to allow more explicitness? This is also allowed:

position = 10%, 10px

To allow users to make layouts that are independent of the output viewport we are missing label:font_size and image:size (which should be image:scale fmpov, but for some reason it was named size...)
But I think it is easier for us to do that in a separate MR.

Part 1 to implement #261
Closes #446 (halgin=left and position = 50%, 0)

[vaxerski]

wiki mr needed