Add colored tabs feature

[vlasix]

Summary

Adds support for custom tab colors.

This allows users to visually distinguish tabs used for different
servers, environments or tasks.

Motivation

Implements the custom tab colors part of #1102.

Implementation

• Tabs can now store a custom background color.
• The color is saved with the notebook layout so it persists when the layout is reopened.
• A tab’s color can be changed from the right-click context via a color picker dialog.
• The selected color is applied to the tab using a CSS class, with a text color chosen for contrast (black or white depending on brightness).
• The tab label also handles button events more reliably so right-click actions work across the clickable area.
• Preserves the existing behavior when no custom tab color is configured.

Tests

• Tested manually on Ubuntu
• python3 -m compileall -f terminatorlib/ tests/ remotinator terminator
• xvfb-run -a pytest

Screenshots