- add error handling for all db operations
- refactoring Database.cc to make it more readable.
  add doc for Database.cc
  extract method copyDB(), initUserDB(), setPragmaOnMainDB()
- fix old timeoutCallback(), now named cb_saveUserDB()
  saveUserDB() should not be in the condition, because save can fail endlessly
  if file system have problem.
- no longer require main db to be a regular file. symlink should work fine.
- drop the dreaded overly reused class variable m_buffer. use meaningful names
  in current context.
- escape string in SQL statements
- update meson.build
  add prefix in PKGDATADIR
  define glib logging domain
  disable glib asserts for release build
- I will use so file version in git log.