Add support for project manager plugins

[me2beats]

Describe the project you are working on:
gdscript plugins

Describe the problem or limitation you are having in your project:
I'd like to create a plugin that would implement Project Templates
#1481
The problem is afaik there's no way to customize Project Manager with gds plugin, my idea was to add there "Project templates" tab.

Describe the feature / enhancement and how it helps to overcome the problem or limitation:
Ability to create custom Project Managers with plugins and choose what Manager implementation to use, could solve the problem.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:
Maybe it could be some kind of plugin that loads MyProjectManager that extends default Project manager class and then uses it instead of default one.
So some API methods would be useful like
— what project manager to use (default or some custom one)
— sort, search, scan projects etc

If this enhancement will not be used often, can it be worked around with a few lines of script?:
No it can't

Is there a reason why this should be core and not an add-on in the asset library?:
This should be core one. Implementing custom ProjectManager with a plugin from scratch seems like a rather laborious affair

[aaronfranke]

This depends on #831 being solved first. Before we can have addons affecting the project manager, we would need addons to be independent from projects (a global addon system). There's also this closed proposal godotengine/godot#17813 which I like better.