Templates: Enable users to quickly browse by author/source

[jameskoster]

When managing templates it can be helpful to quickly view templates by author/source, e.g.:

• Templates added by the active theme.
• Templates added by specific plugins like WooCommerce or bbPress.
• Custom, user-generated page templates ($custom).

To achieve this currently requires several clicks navigating the filters menu.

Dedicated views

This approach builds on the sorting that exists in the current templates list by providing views for the following template groups:

• All templates – self-explanatory.
• Theme – templates bundled with the active theme.
• Custom – any user-generated $custom templates.
• Per-plugin views.

Adding these views could be a nice enhancement for 6.5. The first three should appear regardless, and per-plugin views would appear dynamically when the corresponding plugin is activated.

[jameskoster]

This wasn't fully captured by #58124 so I don't think we can close yet.

Missing details:

• For the 'All templates' view could we use the file icon to differentiate it from the theme views?
• It would be nice to include the counts.
• Perhaps most important: ideally all user-generated templates would be grouped together in a view called "Custom", rather than individual views per user. This view would be the equivalent to filtering by author and selecting all users.

[youknowriad]

Given that RC1 is today I'm removing this one from 6.5 as non essential.

[jameskoster]

Closing this in favor of individual issues for the outstanding items since they're better handled separately.