frontend: Improve plugin manager safe mode behaviour

[Warchamp7]

Description

Allows enable/disable of plugins that are missing in the plugin manager window and adjusts the display of plugins in the list slightly.

Motivation and Context

Currently plugins that are marked as missing have their checkboxes disabled. This is fine when a plugin has been deleted, but this check also includes plugins that were skipped due to being in safe mode.

This means currently you can't disable a plugin when in safe mode, which is arguably one of the primary goals of both safe mode and V1 of the plugin manager.

How Has This Been Tested?

Opened OBS in normal mode and safe mode. Disabled/enabled plugins in both and confirmed plugin state was as expected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.