Reduce sidebar width

Summary:
BUG: 385992
FIXED-IN: 5.12.3

This is a hack to work around the fact that we're using the wrong UI element for Discover's sidebar (see Bug 390928). We use a Kirigami GlobalDrawer for the sidebar, which is problematic because the GlobalDrawer's width is always a minimum of 360 pixels on the desktop--far too wide for our use case (see Bug 390927).

Replacing the GlobalDrawer with something more appropriate is going to be a lot of work, so until then, let's not torture our users with the humongous sidebar any longer and just force it to be a smaller size for now.

I think this is a vast visual improvement because it returns the focus to the content, rather than the navigation. It makes Discover feel less "fat."

Test Plan:
Before, small window:

After, small window:

Before, medium window:

After, medium window:

Before, app page:

After, app page:

Before, default view:

After, default view:

We may want to consider changing the default window size to be less wide if this patch lands, since with a narrower sidebar, the main view doesn't need to be so wide.

Reviewers: Discover Software Store, apol

Reviewed By: Discover Software Store, apol

Subscribers: acrouthamel, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10756