virtualdesktops: Raise virtual desktop limit to 25

BUG:465036

X11 counterpart: kwin-x11!51 (merged)

This is an extension of the work in !6922; per @vladz request, moving the virtual desktop adjustment to its own MR.

Before:

After:

• This would need to be merged before the related Overview MR. Without this bump, the Overview's Desktop Grid effect is capped at 4x5, where 5x5 (25 desktops) perfectly packs the screen. The goal of !6922 is to efficiently pack the Desktop Grid tiles, this is impossible with 4x5.

• Vlad does float the idea of removing the limit, but the OP proposed 25 as a reasonable maximum, and I agree. The limit would prevent a user from accidentally (or intentionally) laying on the + key and running up 100+ desktops.

• This increase won't regress the existing pager/VD presentation. When createVirtualDesktop() is called, this value is checked everywhere except for the Virtual Desktop KCM itself, an existing issue.

  • I've just checked though, and found that is likely due to the KCM + Add Desktop button calling DesktopsModel::createDesktop, and not VirtualDesktopManager::createVirtualDesktop: the former does not check and exit if the maximum is reached. We could try and fix that here or in another MR, it's worth a separate bug report.