Modify plugins to use [Import] to replace singletons

[vchelaru]

@zdman2022 had originally set up the plugins to import their dependencies like this:

[Import("GlueCommands")]
public IGlueCommands GlueCommands
{
    get;
    set;
}

We are moving away from singletons and instead moving to this. Need to start moving some of the main plugins over as examples, and see if there's any work that has to be done to support imports...

Example:

FlatRedBall/FRBDK/Glue/TileGraphicsPlugin/TileGraphicsPlugin/TileGraphicsPluginClass.cs

Line 58 in 5591749

[Import("GlueProjectSave")]

Update 1

It seems like these get imported so long as the PluginManager exports them:

FlatRedBall/FRBDK/Glue/Glue/Plugins/PluginManager.cs

Line 110 in 8ea07ca

[Export("GlueCommands")]

[vchelaru]

Look at importing constructors here:

https://learn.microsoft.com/en-us/dotnet/framework/mef/attributed-programming-model-overview-mef#prerequisite-imports