Added capability of saving inner dialog sizes

[oleg68]

As discussed in #1830 and #1832, now GransOrgue remembers dialog windoow external sizes, but does not remember column sizes.

This PR

• moves all size-related classes to the size subdirectory
• introduces the abstract class GOAdditionalSizeKeeper for storing additional sizes. GOSizeKeeper implements this class.
• introduces methods ApplyAdditionalSizes and CaptureAdditionalSizes to GODialog and GODialogTab. Their successors can override them for (re)storing inner element sizes.

This PR adds a capability for future usage. No GO behavior should be changed now.

[oleg68]

@larspalo I changed Add to Additional and N to Count.