Add flat text entry box

## Problem

In many situations, having the inset text entry box looks off. This is especially the case with renaming items in the Granite.SourceList or in the Files app, where the items are tightly packed and appear flat on the UI by default.
There also seems to be a couple of places something similar is being used, but there is no overarching widget that can be used anywhere.

## Proposal

There should be a Granite.FlatEntry that has custom styling to make the text box not have a box around it, and instead has a rounded 'pill' like accent colour outline for when it is selected.
Another idea may be to automatically put the symbolic pencil icon at the end of the box when hovered to make it obvious it can be edited - and then the symbolic delete icon in the pencil's place while being edited

## Prior Art

Something like this but an actual widget that can be selected with a keyboard