mg-desktop: Reduce excessive disk IO

[jng]

If you look at any process using mg-desktop in the Task Manager, chances are the "I/O Read Bytes" column will be through the roof.

The explanation is simple, the mg-desktop implementation of MgResourceService is a simple wrapper around the file system, so every GetResourceContent() call (and Maestro does a lot of this) is a call to read a file from disk. Such calls add up real quick.

The solution would be to have an in-memory cache of resource content, such that subsequent calls to GetResourceContent() on the same resource id returns a MgByteReader from a cached copy of the resource content. Subsequently, calls to SetResource() will flush existing entries out and update the cached entry.