Add/Update some icons

[ghost]

I have added some icons:

• bleachbit (main icon in 64px to complete the set)
• firefox (main icon)
• synaptic (main and status icons for the GUI)
• keepassxc (main & status icon)
• dropbox (main icon)
• remmina (main icon)
• support for some Gnome apps

Updated utilities-system-monitor to better fit the elementary style.

Hope I got it all right. It's my first pull request :)