Move Plugin/Theme deletion to new WP_Upgrader class

[DD32]

Expansion off #7875, The plugin/Theme deletions should be also handled by the Upgrader class.

Plugin Deletions do a check upfront to verify that deleting the files of one plugin is interfeering some other plugin (same files used for multiple plugins). also plugin deletion is checking for the plugin uninstall api.

for themes i have no idea.

Correct.. Which has absolutely no affect upon this ticket other than to mimic current backend UI.

I do not know wether or not the Upgrader Class is there to mimic backend UI or not. Wether it is part of the (then plugin and theme) API or not.

So my concrete question regarding to this ticket is:

1.) Does the upgrader class needs to handle the plugin API, especially the install and uninstall part of it?

The upgrader class is mearly back-backend, It has a UI class for the UI part of the deal.

Dont worry, I'm not going to remove functionality, Only add or improve the process. (Remember I've written this Upgrader class to remove the old code which i seemed to be the main maintainer of, In the sense that I probably understood it the best, I wasn't happy with the old code.)

I see no reason why the uninstall process cant be handled as well, It'll be left as a separate function, but tied in with the upgrader class. The upgrader class is designed to deal with all the connections and various FS methods which WordPress needs to deal with plugins/themes/custom-objects.