WIP: Global Menu Applet

Summary:
This restores the global menu applet which you can place in panel.

The applet provides two views namely Compact and Full View.
Views can be switched from applet's settings.

• Compact View provides a single button for accessing the application's menu

• Full View shows the full application menu in panel

ToDo
0> compact view's "Menu" button is a hack. should be a circular button with some icon...maybe ?
1> (in compact view) long pressing alt key should trigger the menu
2> (in full view) alt+key should open the relevant menu
3> the keyboard shortcut page in applet's setting is not functional
4> in compact view menu button should grey out when app menu is not available

See https://phabricator.kde.org/D3156

Reviewers: Plasma, broulik, chinmoyr

Subscribers: andreaska, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3706