notebook: Set minimum witdh for tabs

Currently, when there are many tabs, the tab label is fully
ellipsized into an ellipsis.

An ellipsis is not a useful tab label.

Instead, set a minimum of 6 characters for the tab label.

Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=737372
1 job for bz737372-set-min-tab-width in 8 minutes and 12 seconds (queued for 21 minutes)
Status Job ID Name Coverage
  Test
passed #7695
test

00:08:12

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