[gimp] meson: add an `install-icons` meson target.

[Jehan <jehanp src gnome org>]

commit fca64f5f66ea5d6c8549f8d0c4202de325f2fdc2
Author: Jehan <jehan girinstud io>
Date:   Sat Oct 12 13:25:31 2019 +0200

    meson: add an `install-icons` meson target.
    
    We want to be able to install icons only in a quick command when
    testing/developing.
    Also I realize that Legacy icons are not even installed with meson
    build, which is bad! Even though legacy, we want to keep them (at least
    for the time being), just as we do with autotools.

 meson.build | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)
---
diff --git a/meson.build b/meson.build
index 5f7891639b..e82e17bf13 100644
--- a/meson.build
+++ b/meson.build
@@ -1618,14 +1618,15 @@ pkgconfig.generate(libgimpui,
 # Subdir installations
 
 foreach dir : [
-    { 'dir': 'libgimp',   'deps': libgimp},
-    { 'dir': 'libgimpbase',   'deps': libgimpbase},
+    { 'dir': 'libgimp',        'deps': libgimp},
+    { 'dir': 'libgimpbase',    'deps': libgimpbase},
     { 'dir': 'libgimpcolor',   'deps': libgimpcolor},
-    { 'dir': 'libgimpconfig',   'deps': libgimpconfig},
-    { 'dir': 'libgimpmath',   'deps': libgimpmath},
-    { 'dir': 'libgimpmodule',   'deps': libgimpmodule},
+    { 'dir': 'libgimpconfig',  'deps': libgimpconfig},
+    { 'dir': 'libgimpmath',    'deps': libgimpmath},
+    { 'dir': 'libgimpmodule',  'deps': libgimpmodule},
     { 'dir': 'libgimpthumb',   'deps': libgimpthumb},
-    { 'dir': 'libgimpwidgets',   'deps': libgimpwidgets},
+    { 'dir': 'libgimpwidgets', 'deps': libgimpwidgets},
+    { 'dir': 'icons' },
     { 'dir': 'plug-ins', },
   ]
   run_target('install-' + dir.get('dir'),