view-switcher-button: Make an active button's label bold

This makes the view switcher easier to read.

It uses multiple labels with or without the specific style rather than a
single label with the style toggled on and off to ensure the size
requests don't change depending on whether the button is active or not.
7 jobs for view-switcher-bold in 8 minutes and 9 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #29398
librem5
build-debian-clang

00:02:31

passed #29396
librem5
build-debian-gcc

00:02:36

passed #29397
librem5
build-debian-gcc-static

00:01:56

passed #29400
librem5
build-debian-package

00:01:42

passed #29399
librem5
build-fedora-gcc

00:04:04

 
  Docs
passed #29401
librem5
build-gtkdoc

00:01:36

 
  Test
passed #29402
librem5
unit-test

00:02:26

38.8%