[RFC] Reduce size of Level 1 headings and increase left padding on page titles

Summary:
Level 1 headings are humongous. The degree of humongousness is maybe a bit excessive,
especially on page titles with toolbars, where it looks like it's positively trying
to burst from its bounds! Also, the left padding could stand to be increased, which
would make the titles align with the content for most views. This was brought up in
T12631.

If this patch is approved, I will submit companion patches to make the same changes in
the PlasmaExtras Heading and KTitleWidget.

Test Plan:
Discover, before:

Discover, after:

Kirigami Gallery, after:

Reviewers: VDG, Kirigami, manueljlin, mart

Reviewed By: VDG, Kirigami, manueljlin, mart

Subscribers: alexde, ahiemstra, plasma-devel

Tags: Kirigami

Maniphest Tasks: T12631

Differential Revision: https://phabricator.kde.org/D27198