Add Plugin selection tab to collection selection window

[bjsi]

Description

Add a tab to the collection window to choose which plugins I want to load before loading into SM.

Motivation

Useful for devs who want to test a single plugin in isolation without deleting / moving files in the plugin dir. Also means you don't need to stop a bunch of running plugins when you open SMA.

Documentation-type explanation

Simply a datagrid with each row containing a checkbox and the plugin name. Uncheck checkbox if you don't want the corresponding plugin to launch.

[alexis-]

There's already an Enabled field in the Plugin context model. There's also a commented out Toggle button in the Settings's plugin list. It's just a matter of connecting things up and saving the state to disk (in the json).

[alexis-]

FYI, enabled Plugins are now run at startup, immediately after the Setup verification.