Migrate useful exo-string functions to xfce-string

Related: (exo#82 (closed))

I am currently working on moving libexo functions to a more appropriate place. These are renamed functions from exo-string.

Functions

• xfce_str_replace : Find and replace substring
  • Used in thunar, xfce4-settings
• xfce_append_quoted(xfce-miscutils) => xfce_g_string_append_quoted(xfce-string)

Macros

• xfce_str_is_empty : Check if a string is either empty or NULL (Alreay existed in xfce-misctools.h so it is moved to xfce-string.h)
  • Used in xfce4-panel, xfce4-mailwatch-plugin, thunar, xfce4-settings, xfce4-whiskermenu-plugin, exo-icon-chooser-dialog.c, exo-die
• I_ : Abbreviation for g_intern_static_string
  • Used in xfdesktop (redefines the same macro), thunar, libxfce4ui, xfconf (redefines the same macro), mousepad (redefines the same macro), xfce4-settings, xfce4-terminal (redefines the same macro), exo