Fix color entry spamming undo stack (1.4.x)

Irrelevant for 1.5-dev as it uses a new color widget that doesn't have this problem.

Add a static function to only change undo key on color commiting.

There are quite some round-trips between color widgets in 1.4.x, and this is a minimal fix. Let me know if deduplicating code between color widgets and paint selectors is instead preferred.

Tests to match behavior:

for both when entry is active and inactive

• Click on slider: no undo log
• Drag slider: no undo log
• Type in spinbutton: no undo log

---

• Deselect, reselect, click on slider: new undo log
• Deselect, reselect, type in spinbutton: new undo log
• Hold down spinbutton inc/dec button: spamming → no spamming
• Type in hex entry: undo log, spamming → no spamming

before	after

Fixes #5499