Enhancement: Close tabs to the left

[toxpal]

The feature to close tabs to the right (#1362) is super useful for anyone who works with many tabs, so thank you for that.

However, I miss a feature to close tabs to the left badly... It exists in many other editors, and t would be very useful to have the same in Geany (just like we have close to the right, which is already used by many Geany users).