Support new N++ plugin command NPPM_GETTABCOLORID #337

A new notepad member function is needed to support a plugin command new to (presumably) N++ release 8.6.8: NPPM_GETTABCOLORID

The new function should (ideally?) be callable as color_id = notepad.getTabColorID(view, tabIndex) where view and tabIndex are both default arguments with value -1 (to mean active/current view and active/current tabIndex).

Details on the N++ implementation:

• notepad-plus-plus/notepad-plus-plus@9244cd0
• document plugin command NPPM_GETTABCOLORID notepad-plus-plus/npp-usermanual#671

Other related:

• Add plugin command NPPM_GETTABCOLORID notepad-plus-plus/notepad-plus-plus#15142
• [Feature request] Allow plugins to get/set the existing color choice for a tab notepad-plus-plus/notepad-plus-plus#15115