Easier use for other Apps working with Global Sharing API 

Now if you would like to use the global share api so it's much easier to work with!
No additional share js code for an app is needed!
For example e.g. calendar
init share calendar we need:

I. Checking global rights of the share Api
```
$tmpl->assign("mailNotificationEnabled", \OC_Appconfig::getValue('core', 'shareapi_allow_mail_notification', 'yes'));
$tmpl->assign("allowShareWithLink", \OC_Appconfig::getValue('core', 'shareapi_allow_links', 'yes'));
```
II. we need the global input fields
```
<input type="hidden" name="mailNotificationEnabled" id="mailNotificationEnabled" value="<?php p($_['mailNotificationEnabled']) ?>" />
<input type="hidden" name="allowShareWithLink" id="allowShareWithLink" value="<?php p($_['allowShareWithLink']) ?>" />
```
III. we need the share link e.g. sharing Calendar
```
<?php if ($_['calendar']['permissions'] & OCP\PERMISSION_SHARE) { ?>
		<a href="#" class="share permanent" 
		data-title="<?php p($_['calendar']['displayname']) ?>"
		data-item-type="calendar" 
		data-item="<?php p($_['calendar']['id']); ?>"
		data-link="true"
		data-possible-permissions="<?php p($_['calendar']['permissions']) ?>"
		title="<?php p($l->t('Share Calendar')) ?>"
		style="background:url(<?php print_unescaped(OCP\Util::imagePath('core', 'actions/share.svg')) ?>) no-repeat center;"
		   ></a>
	<?php } ?>
```
```
data-title="<?php p($_['calendar']['displayname']) ?>"
```
is new and needed if we would like to send an email to an external!

IV. need init the Icons
```
OC.Share.loadIcons('calendar');
```
That's all!
Now you can manage your shares with the global share api, automatically you can define Share via Link, Send E-Mail internal or send E-Mail for external User!

Please revert this.
I think it was a design decision to use the other icon. CC @jancborchardt