fix(ui5-messagestrip): Close button now has the correct design

[vladitasev]

Changes to ui5-button to support this change:

• new property iconSize added, allowing the user control not only over the button size, but over the icon size as well
• padding removed for iconOnly mode, icon is now centered

Buttons like this are now possible:

with the following markup:

ui5-messagestrip was refactored:

• ui5-button used to achieve the desired design and behavior for the close button (kb handling, hover, focus, etc..)
• CSS simplified (same rules merged, duplicate selectors removed, redundant CSS removed after the close icon removal, etc...)
• CSS classes renamed and reordered
• keyboard handling logic and all code that simulates ui5-button behavior deleted
• onDefine now awaits all resources simultaneously

The close button looks like:

closes: #1989

[ilhan007]

I am personally ok with the change, but the close button should be smaller in width by spec, now it became bigger with the ui5-button

[ilhan007]

I think @MapTo0 added this css for some IE reasons in the past