main: Refactor quit functions

Move quit-related functions to main.c and make them static. main_quit() now
does all the heavy work and can be called from signal handlers.
Update Scintilla to version 3.4.4
Use ui_button_new_with_image() to build image buttons
Add tooltips to explain the purpose of the New Project dialog fields
Ask whether to overwrite the project file when creating a new project
Custom commands dialog: Make sure the selected item is always visible

Always bring back the selected item after an user action, so the user
can see what she's doing.
Import libpeas and integrate into build system
peas loaded plugins information
actually load plugins with libpeas, integrated into PM dialog
tmp