New functions and configurable keyboard shortcuts

* Also added "General" and "Shortcuts" settings tab.
* Made all keyboard shortcuts configurable
* Added option "Restore tabs"
* Added option "Single instance"
* Added function "Delete line(s)" 
* Added menu entry and keyboard shortcut option for: 
  * "Save All" 
  * "Find Next" 
  * "Find Previous" 
* Function "Go to line" reacts on Return key
* Removed feature "compile unsaved save". Compiling unsaved files appear unsaved in the front end. 
* Changed handling of config read error: Silently discard error and continue reading. 
* Set focus on start to text input 
* Added opt-in feature "Save all on compile" 
* Changed main window title to "file - Aporia" (saved) or "file* -Aporia" (unsaved)
I don't understand why "gtk2.delete(textBuffer, ...)" works but not textBuffer.delete(...).
I kept this line so I can keep an eye on this issue.