[KCM] Scale more coarsely with the slider, but more finely with a spinbox

Summary:
Right now we have a problem in that the important scale factors of 1.25 and 1.75 are
not reachable using the UI. However just reducing the slider increment to 0.05 would
result in way too many slider values. Instead, this patch implements the following:
- The slider goes by increments of 0.25
- There's a spinbox beside the slider that allows the user to set the scale factor by 0.05 increments
- On X11 with global scaling, the user is shown a warning message when using scale factors that are not a multiple of 0.25

This way the commonly-used scale factors are more accessible, but finer values are
made available to people who really need them and are likely to go poking around.

BUG: 412447
FIXED-IN: 5.18.0

Test Plan: {F7489713}

Reviewers: #vdg, #plasma, romangg, ndavis

Reviewed By: #vdg, #plasma, romangg, ndavis

Subscribers: dhaumann, davidedmundson, ouwerkerk, GB_2, ndavis, cullmann, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24321