Give more choice for whitespace display

Depends on !48 (merged).

By addressing #44 (closed), this MR extends whitespace display to what GtkSourceView 3.24 allows (so bumping its version in configure.ac.in):

• four white space types: space, tab, newline, non-breaking space;
• three white space locations: leading, trailing, between words.

To achieve this, I chose to use a GtkPopover in the preferences dialog, built from a GMenuModel whose actions lie in MousepadApplication (since these settings make sense at the application level). This is what it looks like:

The binding with GSettings is done also in MousepadApplication, while the view properties are bind to the application properties (one for the space type flags and one for the space location flags).