Fix misaligned Entry widget borders
When a entry is part of an horizontal linked box, in a headerbar, it's borders appears misaligned.
As examples, see Image Viewer's zoom selector or Evince's "page X of Y" widgets.
This commit adjust borders, margins and padding in order to re-align the borders of the widget:
- When the entry is squared (Image Viewer), the rounded border-image is removed and replaced with the appropriate border-width and color @focus_color, when on focus
- When there are two entries joined together (Evince), the margins and borders are adjusted to match the given border-image in status "normal", focus, disabled and disabled:backdrop (LP: #1712996)
Approved by: Marco Trevisan (Treviño)