applications: Move default app and removable media

Currently setting's panel list is over-populated. We can fix this by moving some of the settings inside other panels as a sub-page.

First, updated applications panel to conform to latest design mockup.

Link: https://gitlab.gnome.org/Teams/Design/settings-mockups/-/blob/master/applications/applications-wires.png

Second, created a new row in applications panel for default apps and removable media. On selecting the row we show the respective settings.

The default app and removable media sections are moved from their previous location. Default App and Removable Media Panels don't exist.

Closes: #1092 (closed) #1096 (closed)

While it is working fine now, the following might be a problem

• As default apps and removable media have been moved under applications they aren't searchable anymore (desktop files don't exist). This might cause confusion for people used to it previously.