Add GNOME 50 compatibility

[SMARTYFRR]

Summary

Adds GNOME 50 compatibility fixes for Open Bar.

Changes

• Add GNOME 50 to the supported GNOME Shell versions.
• Fix GNOME 50 message-list actor traversal.
• Fix panel margin specificity for GNOME 50.
• Add a null check when applying the clear-button style.
• Update the README with GNOME 50 installation instructions.

Testing

Tested locally on Fedora running GNOME 50.

Verified that Open Bar:

• Loads successfully on GNOME 50.
• Applies Floating mode correctly.
• Applies Trilands mode correctly.
• Applies panel margins correctly.
• Applies panel height changes correctly.