More small improvements for output configuration store
Commit 1:
outputconfigurationstore: only round the scale factor to 5% steps
This is the same step size as the spinbox in kscreen, and allows the default scale to
get closer to the ideal one we're actually targeting.
Commit 2:
outputconfigurationstore: increase the max. scale when reading the config
While we (currently) don't allow setting more than 300% in kscreen, there are some edge
cases where more than that is sensible.
This commit bumps the limit to 500%, which is more than has been requested by anyone so far.