Add option to show text on taskbar buttons for a more classic taskbar.  ...

...Takes some code from https://github.com/zpydr/gnome-shell-extension-taskbar/pull/31/files

Fixes the merge problems with @ionuke's code, and adds logic to intelligently size the taskbar buttons so that they have a minimum/maximum width and fill all the space available to the left panel.

The setting can be enabled in the preferences->tasks->text labels on task slider.