feat(ui5-dialog): introduce draggable property

[georgimkv]

Part of #2082

[CLAassistant]

All committers have signed the CLA.

[ilhan007]

Thanks for the PR. It works fine, I tested the change in IE as well, you can find few minor comments in place, after that we can merge the PR

[ilhan007]

We should also apply the correct cursor when the user sets a "header" slot.
Because this CSS applies for the built-in header that shows the "header-text", but doesn't apply to the user defined header via the "header" slot. Explore the ::slotted CSS selector in order to make it.

[georgimkv]

I added it, but the ::slotted selector doesn't allow selecting inner elements deeper in the Shadow DOM, only the most top level inner elements. Because of this, I can't reset the cursor of the inner children like I am doing with the rule

:host([draggable]) .ui5-popup-header-root * {
     cursor: auto
}

This results in elements such as headings still inheriting the "move" cursor. A ui5-button, for example, will have correct cursor "pointer", because its own CSS defines it.

[ilhan007]

Also, found some strange side effect in IE. At the moment someone start dragging the dialog in IE, the content gets a scrollbar (did not have it before), without a reason.
*It can be reproduced with the dialog with the "draggable" attribute set in the Dialog test page in IE11.

[georgimkv]

Fixed the IE issue. Was caused by inconsistent number rounding.

[ilhan007]

Thanks for addressing the comments. Now the draggable test is failing, could you take a look of it.
Regarding the cursor pointer of the header slot, we can live with that. But, If we receive an issue in future we should spend time to find a way to resolve this.

[georgimkv]

Thanks for the review Ilhan. I've amended the Dialog.html to keep the visual test straightforward. I will follow up with another change for the other user story on the issue.