Bob Bai authored Jan 06, 2020 and Nate Graham committed Jan 06, 2020

Some discussions at https://phabricator.kde.org/D25484.

This MR adds:

* mouse middle button event handler for closing a tab
* an `undo close tab` action

Test plan:

1. Compile and install.
2. Enable `Open new files in tabs`.
3. Open two documents. They should be opened in two tabs in the same window.
4. Middle click one of the tabs. It should be closed.
5. Choose `File`-`Undo close tab`. The closed tab should be reopened.
6. Middle click one of the tabs. It should be closed.
7. Press <kbd>Ctrl</kbd>+<kbd>Shift</kbd>+<kbd>T</kbd>. The closed tab should be reopened.