Implement GTK+ GUI config tool

[kakurasan]

This PR provides GTK+ GUI config tool for Redshift. The tool (redshift-config-gtk) outputs redshift.conf.

Screenshots

General

Time

Advanced

Notes

• This tool is written in C because it needs to directly use some of Redshift's source files
  • gtk+-3.0 is required when building
• The tool changes gamma ramps
  • Multiple instances are not allowed
  • redshift (continual mode) and redshift-config-gtk shouldn't be used at the same time
  • No changes in redshift-gtk because currently redshift-gtk cannot "quit redshift, launch this tool, and restart redshift to apply changes without restarting redshift-gtk"
• The default config directory is determined using g_get_user_config_dir()
  • On GNU/Linux, if XDG_CONFIG_HOME is defined, the tool writes to ${XDG_CONFIG_HOME}/redshift/redshift.conf
    • If undefined, (home directory)/.config/redshift/redshift.conf is used
• It doesn't preserve comments in existing redshift.conf
• -c and -m options can be used
• Translatable
  • Main source: src/redshift-config-gtk/redshift-config-gtk.c
  • Menu XML: src/redshift-config-gtk/gtk/menus.ui
  • make update-po is needed after merging this PR