Add optional delete from disk in collection and playlist

Fixes #284

src/settings/collectionsettingspage.cpp

@@ -178,6 +178,8 @@ void CollectionSettingsPage::Load() {
   ui_->spinbox_disk_cache_size->setValue(s.value(kSettingsDiskCacheSize, kSettingsDiskCacheSizeDefault).toInt());
   ui_->combobox_disk_cache_size->setCurrentIndex(s.value(kSettingsDiskCacheSizeUnit, static_cast<int>(CacheSizeUnit_MB)).toInt());
 
+  ui_->checkbox_delete_files->setChecked(s.value("delete_files", false).toBool());
+
   s.endGroup();
 
   DiskCacheEnable(ui_->checkbox_disk_cache->checkState());
@@ -227,6 +229,8 @@ void CollectionSettingsPage::Save() {
   s.setValue(kSettingsDiskCacheSize, ui_->spinbox_disk_cache_size->value());
   s.setValue(kSettingsDiskCacheSizeUnit, ui_->combobox_disk_cache_size->currentIndex());
 
+  s.setValue("delete_files", ui_->checkbox_delete_files->isChecked());
+
   s.endGroup();
 
 }

src/settings/collectionsettingspage.ui

@@ -450,6 +450,13 @@ If there are no matches then it will use the largest image in the directory.</st
      </layout>
     </widget>
    </item>
+   <item>
+    <widget class="QCheckBox" name="checkbox_delete_files">
+     <property name="text">
+      <string>Enable delete files in the right click context menu</string>
+     </property>
+    </widget>
+   </item>
   </layout>
  </widget>
  <tabstops>

src/settings/playlistsettingspage.cpp

@@ -83,6 +83,8 @@ void PlaylistSettingsPage::Load() {
   ui_->checkbox_editmetadatainline->setChecked(s.value("editmetadatainline", false).toBool());
   ui_->checkbox_writemetadata->setChecked(s.value(Playlist::kWriteMetadata, false).toBool());
 
+  ui_->checkbox_delete_files->setChecked(s.value("delete_files", false).toBool());
+
   s.endGroup();
 
   Init(ui_->layout_playlistsettingspage->parentWidget());
@@ -118,6 +120,7 @@ void PlaylistSettingsPage::Save() {
   s.setValue(Playlist::kPathType, static_cast<int>(path));
   s.setValue("editmetadatainline", ui_->checkbox_editmetadatainline->isChecked());
   s.setValue(Playlist::kWriteMetadata, ui_->checkbox_writemetadata->isChecked());
+  s.setValue("delete_files", ui_->checkbox_delete_files->isChecked());
   s.endGroup();
 
 }

src/settings/playlistsettingspage.ui

@@ -63,6 +63,13 @@
      </property>
     </widget>
    </item>
+   <item>
+    <widget class="QCheckBox" name="checkbox_delete_files">
+     <property name="text">
+      <string>Enable delete files in the right click context menu</string>
+     </property>
+    </widget>
+   </item>
    <item>
     <widget class="QGroupBox" name="groupbox_paths">
      <property name="title">