add possibility to add custom button to title widget

[MichaelVoelkel]

I'd like to have one or multiple custom buttons added to the title widget, probably left to the menu button.

It could be put into the title widget but say we have 5 buttons that could apply to any of the multiple tabs. This would bloat the menu since they would be multiplied by the number of tabs. So, for 3 tabs and 5 buttons, we'd actually had 15 buttons, although 5 would be sufficient. Writing an answer here at Github shows the same system: Write and Preview tabs are available but all the tool buttons like bold, italic, quote, ... are common for both of those tabs.

[mfreiholz]

Sounds very good :)