[sddm-theme/lock screen] Overhaul font sizes

Summary:
This patch:
- tells the login screen to modify the font size specified in theme.conf and the lock screen to modify the user's default theme's font size instead
- makes sure that the login screen session and keyboard buttons take this new approach too
- raises the font size of "Start New Session" on the lock screen (to match the rest)
- raises the artist/track font sizes (to match the password field)

Test Plan:
This patch resolves the problem of

(1) The fonts on the login screen and

{F6676520}

(2) the fonts on the lock screen

{F6676524}

not having matching sizes.

Reviewers: #vdg, #plasma, ngraham, filipf

Reviewed By: #vdg, ngraham, filipf

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Maniphest Tasks: T10325

Differential Revision: https://phabricator.kde.org/D19631