[Grid] Implement frozen first column for SWT (RCP) parity with RAP

[PHMT-CW]

Problem

GridColumn#setFixed has existed in Nebula Grid since PR #645 but is only honored by RAP. In the SWT RCP version, scrolling horizontally still slides the "fixed" column off-screen, and a long-standing TODO in Grid.java acknowledges this gap.

Proposal

Make the SWT implementation honor setFixed(true) for the first column:

• Paint a fixed overlay pass for header, cells, and footer that stays put while the rest scrolls horizontally.
• Route hit-tests (getColumn, getCell, header/footer hover, mouse handlers) to the frozen column so expand/collapse, editors, and selection all work while scrolled.
• Handle GridColumnGroup spanning: detect a fixed column that shares a group with scrollable members and degrade gracefully with a one-time warning.

Scope is intentionally limited to a single frozen first column to keep the change focused and reviewable. Will supersede the TODO and extend the work started in #645.

Willing to contribute

Yes — PR incoming.