Allow to associate project categories to colors

The color is then used as background in the board and list views

app/Template/category/edit.php

@@ -11,5 +11,8 @@
     <?= $this->form->label(t('Description'), 'description') ?>
     <?= $this->form->textEditor('description', $values, $errors, array('tabindex' => 2)) ?>
 
+    <?= $this->form->label(t('Color'), 'color_id') ?>
+    <?= $this->form->select('color_id', array('' => t('No color')) + $colors, $values, $errors, array(), 'color-picker') ?>
+
     <?= $this->modal->submitButtons() ?>
 </form>