Mutual Checker, Show Originals: Combined custom dashboard tab functionality

[marcustyphoon]

Concept

One inserted tab bar. The util provides the "All Posts" tab, features can register additional tabs (inserted to the left). If there are no additional tabs registered, the util removes the tab bar and becomes inactive.

CSS would need to be adjusted to align text to vertical middle for better mobile compatibility.

We don't have to figure out how to make this scrollable when the maximum number of tabs is three. XKit Rewritten is a packaged add-on, so we don't need to build extensibility until we need to use that extensibility.

Originally posted by @AprilSylph in https://github.com/AprilSylph/XKit-Rewritten/issues/1972#issuecomment-3502428036

[marcustyphoon]

Note that this presumably would involve saving the user's remembered tabs in the utility (inheriting from Show Originals). Effectively this is moving a not-insignificant amount of Show Originals code into a util.