DesktopEntry item

[dnalor]

• added DesktopEntry class to create a menu item from a .desktop file
• create_appmenu_flat /create_appmenu_categories function to create a menu from a directory of .desktop files (flat or hierarchical by app category)

[miphreal]

@dnalor, thanks for your contribution

The idea of generating menu based on *.desktop files definitely makes sense.

I'll be quite busy this week, sorry in advance that the code review may take longer than expected.

Meantime I've got a couple suggestions:

• would be great to use spaces instead of tabs (ideally, would be great to apply black code formatter to rofi_menu/contrib/desktopentry.py)
• we can use python's glob or even better Path.glob for walking through the file tree

[dnalor]

While I can easily replace os.walk with Path.glob, I'm not sure I see the point - a quick benchmark suggests Path.glob is slower.
Finding all *.h files in /usr/include and /usr/local/include:
walk: 10494 files: 0.072s / glob: 10494 files :0.194s

Admittedly, for this use case performance is probably irrelevant, but still...