Avoid deleting custom content when updating/uninstalling

[FeXoR]

When the game is re-installed (for example updated to a new version) the custom made scenarios/random map scripts/AI scripts should stay in place. If this place changes between versions the installation process should check for content in old paths and move the content where it is loaded in the new version.

Currently custom files are deleted during update (I guess uninstalling) process.

I added it as a release blocker because it effects non-svn-users mainly.

I don't know to witch component it belongs.

[FeXoR]

It's related to Ticket #1145 but the impact was underestimated IMO.

[historic_bruno]

This is really just a side effect of having incomplete mod support. The data files that get installed shouldn't be modified by the user, instead we'll add a user mod directory in My Documents (files placed there will override the default mod). The VFS will need some changes to make that happen. #1145 is one step toward implementing the user mod concept.