The TabBar itself does not stretch the entire width of the TabWidget,
because it leaves space for the width of the new-tab button. So, we
manually tell Qt to paint the TabBar's background into the gap.
Fixes#768.
Makes use of the fact that QTabWidget automatically reserves space
for "corner widgets", and gives us the ability to override their
location with a custom style.
Also, determine the height of the new-tab button using the height of the
tab bar, instead of a hard-coded 32px which was too tall on MacOS.
This shows the following actions:
* Reload Tab
* Duplicate Tab
* Move Tab
* Move to Start
* Move to End
* Close Tab
* Close Other Tabs
* Close Tabs to Left
* Close Tabs to Right
* Close Other Tabs
By default, Qt will grow the width of a tab button to fit the title text
of the tab. For long titles or file:// URLs, this looks rather bad. This
sets a min/max tab width to prevent such infinite growth.
To do this, we have to subclass both QTabWidget and QTabBar, because the
functions to be called/overridden are protected.
