Add a setting to hide/remove close ("x") button from tabs

Issue Details:
Given that the size of tabs can change depending on each tab's title, attempting to use the mouse to change the current tab is fraught with danger: if a title suddenly changes, then you may end up hitting the close-tab button instead of switching.

Fix Details:
A possible solution to this is an option (disabled by default) to hide the close button for all tabs: users can still close tab with middle-click or Ctrl+W (or whatever hotkey they've set), but there is no risk of accidentally hitting the close button, with the added bonus of reducing the size of each tab.

NOTE: There are a number of other possible fixes for the problem I've described above (including "Restore closed tab" #960, "Confirmation prompt when closing a tab" #2976 and "Keep the width of tabs the same while the user is hovering over the tab bar"), but those are out-of-scope for this request.