Move letter-spacing, word-spacing, etc. spinbuttons to popover.
The text toolbar is way too wide. The character positioning spin buttons contribute the most to the width. Currently, when not shown one can use the triangle at the end of the tool bar to access an "overflow" menu that has sub-menus where predetermined values can be set for these properties/attributes. The values proposed are very limited and almost certainly not what what users would want.
This merge request is an attempt to offer users a better experience by making the spin buttons available from a "popover" which will stay open until the "popover" button is clicked a second time.
Note that in GTK4, the "overflow" menu is not available so we will need to deal with too wide tool bars.