feat(cosmic-panel-button): add capability to execute commands

[Quackdoc]

Adds the ability to run commands directly from the app instead of needing to create a separate .desktop file an example invocation of this would be below

A couple things I want to fix up,

• The if statement is stupid and rustfmt did it no favors, however I ran out of time.
• I don't actually know what the icon does since setting it has never actually done anything for me
• word the help better or maybe just remove it and put it into it's own little document file

as it stands however i'm out of time until next week or so.

[Desktop Entry]
Name=Cosmic Panel wvkbd trigger
Comment=Applet to trigger wvkbd
Type=Application
Exec=sh -c "cosmic-panel-button --exec 'notify-send cosmic-panel-button_pressed' --name 'send-notification' --icon bell"
Terminal=false
Categories=Cosmic;Iced;
Keywords=Cosmic;Iced;
Icon=bell
StartupNotify=true
NoDisplay=true
X-CosmicApplet=true