feat(ui5-button): add second (end) icon

[NHristov-sap]

Button now can have second icon at the end

BREAKING CHANGE: iconEnd property is changed from boolean to string type and now can accept name for second/end icon.

Before:

<ui5-button icon="home" icon-end>Button</ui5-button>

Now:

<ui5-button end-icon="home">Button</ui5-button>

or

<ui5-button icon="employee" end-icon="home">Button</ui5-button>

[vladitasev]

I think the icon-end property must be deprecated and a new end-icon property added. The icon-end name has boolean semantics ("icon is at the end" - true/false) while the end-icon name would mean "icon which is at the end" or "end icon" (noun). Same for the private property: has-icon-end -> has-end-icon

Then, it would be more straightforward for users, f.e. <ui5-button icon="employee" end-icon="add>Test</ui5-button>

Avatar.ts has for example fallback-icon (the word "icon" is again at the end)

[NHristov-sap]

I think the icon-end property must be deprecated and a new end-icon property added.

I hope you mean 'icon-end' to be completely removed, as if it stays (even deprecated), and we introduce new property 'end-icon', there might be situations where there are two icons at the end. Also, technically it will be very difficult to maintain both options at the same time due to the way 'icon-end' works now.