also moves gtk flags into separate variables for cleanliness

By default, a button will display the relative path to a file within
the current working directory, if it is in there, and otherwise
will display an absolute path.

With the -f/--name-only option, only the basename will be displayed
for any file.

With the -F/--full-path option, only absolute paths will be used for
any file; this was the default previously.

When adding a file button and enabling the filename only option, the
`filename` variable becomes unsuitable for obtaining the pixel buffer
from the file.

The tooltip needs to display the file's path, but will be unavailable if
the label is passed as the basename only. If the tooltip value is null,
default to the label input.