feature(ui5-messagestrip): allow any content for the icon

[vladitasev]

• The icon property is now a slot, therefore the user must provide the icon directly
• This also allows for other content (such as an animated gif) to be used instead of an icon: see the samples
• The noIcon property is unchanged - it still takes precedence over the icon, no matter that it is now a slot
• The positioning of the icon is now done via a placeholder div, rather than by applying a class directly on the icon

BREAKING CHANGE: The icon property is now replaced with the icon slot. If you used a custom icon, for example:
<ui5-messagestrip icon="palette">
you should now pass an icon instance directly:
<ui5-messagestrip><ui5-icon slot="icon" name="palette"></ui5-icon>

[fifoosid]

If the user can pass everything, we can think of a more generic name. Something like graphicalContent or visualContent

[vladitasev]

I think icon is fine, because it is consistent with all other components. It implies that you are expected to pass an icon normally, but as it is HTMLElement, you can pass anything technically.