view-switcher: Define a minimum natural width

This prevents the buttons from looking terribly narrow in a wide bar by
making them request a minimum good looking natural size.
7 jobs for view-switcher-bar in 13 minutes and 10 seconds (queued for 39 seconds)
Status Job ID Name Coverage
  Build
passed #38965
librem5
build-debian-clang

00:02:17

passed #38963
librem5
build-debian-gcc

00:02:35

passed #38964
librem5
build-debian-gcc-static

00:01:52

passed #38967
librem5
build-debian-package

00:01:46

passed #38966
librem5
build-fedora-gcc

00:02:36

 
  Docs
passed #38968
librem5
build-gtkdoc

00:01:33

 
  Test
passed #38969
librem5
unit-test

00:02:44

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