[Notification] add new version of notification widget

[mathildemerle]

This PR continues the complete change of medInria notification system (which was in a bottom bar).

This PR adds the class of the notification widget itself, displayed on middle top of the application, and which close automatically after x seconds. It can be closed by the user manually also if he/she wants. The idea is to use it to display quick information to the user, not full paragraphs. The notification has 3 types: MESSAGE for quick info, SUCCESS for success messages, and ERROR for error messages.

To test

• add in medHomageArea.cpp for instance include <medNotificationWidget.h>
• add medLog in target_link_libraries from its CMakeLists.txt
• in medHomepageArea::resizeEvent for instance (just after the application has its final size), choose the type of message you want to test:

    auto notification = new med::NotificationWidget(this);
    //------------- Message test
    //notification->setText("A longer message to display a tip or information to the user");
    //notification->setIcon(med::NotificationWidget::MESSAGE);
    //------------- Success test
    //notification->setText("Segmentation succeed");
    //notification->setIcon(med::NotificationWidget::SUCCESS);
    //------------- Failure test
    notification->setText("Segmentation failed, this tool needs a mesh as input");                          
    notification->setIcon(med::NotificationWidget::ERROR);
    //------------- Show widget
    notification->show();

Screenshots of example

PS: no need to review the svg lines. Those are images, and Github does not understand them as such.
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