Wrapped widget enhancements

• added methods and callbacks for expansion and collapse of rows in TreeView
• ability to set color of Button widgets
• add ColorDialog, a color selection dialog
• add FileDialog, a file/directory dialog (can replace FileSelection)
• fixed an issue with custom styling being inherited unnecessarily

Fixed a bug where FITS loader preference on command line options
or configuration file was not working

Add ability to dynamically reload separately distributed plugins
using the Command plugin (with reload_local or reload_global commands)

Changed default port for Remote Control plugin and made a command line
switch (--rcport) to allow launch time changing of the port

Fixed an spurious error when closing Ginga after shutting down the RC
plugin manually

A fix for the Tk widget on Windows and Mac OSX to allow proper scrolling

Better formatting of cuts and plot limits (limit excessive decimals)

Fixed an error in reference viewer with detection of Gtk toolkits

Gtk4 backend support is much improved

Enhancements to the thread pool, with dynamic adding of threads when
busy and stopping of threads when they have been idle