Note: Slot 1 X opens gem picker instead of removing main skill

This behavior was introduced in 6c3e9d6 ("Disabled level and quality
for support gems") but is not reflected in that commit's message.
Adding this marker commit for reviewer/history clarity.

What it does:
- Clicking the X button on slot 1 (main active skill) no longer removes
  the gem; instead it opens the GemSelectControl dropdown so the user
  can change which active skill occupies the main slot.
- Tooltip on slot 1's X button reads "Change the main skill gem."
  (instead of "Remove this gem.").

Rationale: treats the main-skill slot's X as "change" rather than
"delete", since the main slot is conceptually always populated.

Author: marckaraujo