Fix room header members icon not filled when enabled

[GimleLarpes]

Description

The current behaviour of the "Members" button in room headers is incorrect, as it is not filled when the members drawer is open.

This PR makes it so that the "Members" button is filled when enabled, as is the correct behaviour according to [1997kB].

Fixes #

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [o] New feature (non-breaking change which adds functionality)
• [o] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [o] This change requires a documentation update

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings