Split panes resize trigger size

[akhuoa]

The splitpanes__splitter width and height are 1px, making it hard to mouse over and drag to resize the panels.

The width and height (for both horizontal and vertical) should be increased so that mouse hover will show a resize pointer, but the border line remains 1px.

• The position of the resizer should be able to store and add to the permalink.

[akhuoa]

The updated splitter area is available for testing.
https://mapintegratedvuer-mapcore-754fcb3bf891.herokuapp.com/#/

The pull request has been created, but still adding more improvements.
#479

[akhuoa]

This feature should work with the permalink. E.g., to retain the panel's size.