Branding: Add Discover / Software Center as a Favorite in the menu and taskbar

[TraceyC77]

Branding: Discover / Software Center on all DEs
Add ask a Favorite in the menu
Add to taskbar

[TraceyC77]

The software centers have already been added to the ISO generation

[EbonJaeger]

Branding trackers:

• Budgie: getsolus/budgie-desktop-branding@c3a89e8
• GNOME: schemas: Swap solus-sc for GNOME Software gnome-desktop-branding#27
• Plasma: N/A (We set this in our plasma-desktop package)
• Xfce4: N/A (We set this in our xfce4-panel package)

Package trackers:

• Budgie: Pending
• GNOME: Pending
• Plasma: plasma-desktop: Swap solus-sc for KDE Discover #6338
• Xfce4: xfce4-panel: Swap Software Center for Discover #6337

[EbonJaeger]

As a note, I think we should wait until just after the Epoch to merge the actual package changes. I don't think we want to swap out the pins beforehand, not knowing if a user has installed a new software center or not. I guess there is a window where it's going to be broken either way, but I feel like this minimizes the window.