Fix the DropDown list when too close to the right screen edge

[dimonomid]

There was a bug: DropDown always aligns the left edge of selection list and the main widget, but when the widget is too close to the right screen edge, the list ends up being drawn only partially. See screenshot: here, "Menu" is the dropdown:

This is how it looks after the fix:

When there is enough room on the right side, dropdown list is drawn exactly as it used to.

Fixes #726.