feature: allow cpAlphaChannel to be changed after initialization

[ExpiviRick]

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [X] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Documentation content changes
• [ ] Other... Please describe:

What is the current behavior?

Opening the color picker dialog will use the initial cpAlphaChannel value. Assigning it to a variable will not work.

Example:

<input [value]="color"
       [style.background]="color"
       [cpAlphaChannel]="alphaEnabled ? 'always' : 'disabled'"
       [cpOutputFormat]="'hex'"
       [(colorPicker)]="color"/>

What is the new behavior?

The color picker dialog will update its cpAlphaChannel before opening. This will make the cpAlphaChannel option more reactive. This allows the color picker to be used with a dropdown / checkbox which decides whether alpha should be enabled/disabled (on the same page).