Editor: configurable sprite numeration in workspace settings and sprite folders

[ivan-mogilko]

Problem

This is written after discussion in #463.

Sprites have numeric IDs in AGS, and that's not something that may be changed without a big overhaul, because this identification is deeply rooted in the engine, it's data formats and it's API. So I will not discuss this here.

It may be sometimes wanted to restrict the range of IDs given to newly imported sprites though. These cases are:

1. Better organization. While AGS allows to change sprite ID after import, that might be a tedious task to do by hand. Actually this may be improved too, but this is not always safe, as sprite numbers may be found in game properties and script too (or even custom game files).
2. Team work and merging work done by multiple people in parallel (or same person, but in parallel steps). This was the primary reason to write this ticket. Problem is, when imported sprites receive IDs from the same range, and this is done in several project states in parallel, the conflicts are imminent.

Proposal

1. Implement "sprite range" setting for the sprite import operation. The new IDs are retrieved from this range. Default range is max supported by the engine (which is 0 to 89999 at the time of writing). When there is no free slot remaining in the range, import operation fails with a message explaining the reason.
2. Allow to set "sprite range" for the workspace. We have workspace settings saved as "Game.agf.user". But we do not have a dedicated UI for such settings. So that should be added, maybe as another node in the Project Explorer, or alternatively as another tab on General Settings. Or else.
3. Allow to set "sprite range" for each Sprite Folder. AFAIK these do not have any properties yet, but it should be possible to support them, and let edit these in Properties Grid as usual. The Sprite Folders are part of the game project, so shared, but team members may make a rule for each person to only import sprites into particular folder(s).

This should allow for the team members to organize the sprite numeration for each member or for each group of sprites, and merge their work more easily.