contrib: Add pulseview.desktop file

A .desktop file is used to integrate a desktop application with the desktop
environment. For example, in KDE, this makes the application visible in
the Launcher, under Applications.

This is the .desktop file we use in Fedora, but it is usable by any distro.
Added All Files to the open file filter list
Initial support for input file formats
Make SigSession release it's device on destruction
Create signals when device is selected
Link signals to probes
Added empty context bar
Derrived Signals from SelectableItem
Derrived TimeMarker from SelectableItem
Moved Signal selection behaviour into SelectableItem
Moved highlight pen into SelectableItem
Added selection highlights to Cursor
Added pv::view::MarginWidget as a common base class of Header and Ruler
Added clear_selection to Ruler and Header
Added selection_changed signal to Ruler and Header
Add selection to Ruler items
Added selection_changed signal to pv::view::View
Wrapped Cursors in a shared_ptr
Removed TimeMarker copy constructor
Added get_context_bar_actions