view-switcher-button: Use buttons borders in size

When computing the size of the button, take the button's border into
account.

Fixes Librem5/libhandy#108
7 jobs for view-switcher-button-border in 7 minutes and 52 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #35212
librem5
build-debian-clang

00:02:43

passed #35210
librem5
build-debian-gcc

00:02:49

passed #35211
librem5
build-debian-gcc-static

00:02:07

passed #35214
librem5
build-debian-package

00:01:44

passed #35213
librem5
build-fedora-gcc

00:02:36

 
  Docs
passed #35215
librem5
build-gtkdoc

00:01:30

 
  Test
passed #35216
librem5
unit-test

00:02:28

40.0%